Gotta catch 'em all!

This thread is dedicated to those Pokémon you want so much, but have no idea where to catch.
In this thread, all your catching worries will be a thing of the past.
Simply post which Pokémon you are looking for and which Pokémon game, and the answer will be presented very quickly indeed. Every single Pokémon's location will be told, if needed. All 493*. Even the ones you can only get through glitches (we cannot say any codes via cheat cartridges, as it's cheating by definition, and we cannot endorse cheaters on tSD.).
*They will be more once Black and White are released.

---Pokémon nests---
#151 - Mew (B, R & Y) : Nugget bridge, Mew glitch (see below (Opening post)).
#251 - Celebi (G, S, C) : Route 34 (Egg), Celebi Egg glitch (Page 1)

---Pokemon Capture hints---
Roaming Pokémon (G, S, C, Ru, Sa, E, LG, FR, P, D, Pt, SS, HG, B, W) : Various Areas (Page 1)

So, ask away! (:


"Mew Glitch" - Pokémon Red, Blue and Yellow
1. Make sure you have an Abra any other Pokémon that knows TELEPORT, or a Pokémon that knows FLY.
2. Make sure you haven't fought the trainer in the patch of grass, just left of Nugget Bridge.
3. Make sure you haven't faced any of the trainers in Cerulean City Gym, yet.
Now this bit a little tricky, so you'll need save just before during it, to make sure you don't mess up by mistake; Go above, and just outside the Trainer just left of the bridge with the Pokémon that can Fly or Teleport, then take one step down and quickly press start. If done right, that trainer wouldn't of noticed you yet. Then, use TELEPORT (or FLY), to take you back to Cerulean City's Pokémon centre. The trainer should notice you just before you fly/teleport away, thus avoiding battle. Now, as you'll probably notice, you might not be able to go to menu, that is because the game thinks you're in a battle.
So anyway, go into Misty's gym and battle any trainer there and beat them. Now finally, go to Nugget bridge. Once you go far enough up, you'll notice your menu will suddenly pop-up, once it does press start, and you should be in a battle with a level 7 - level 15 Mew now. Now it's up to you to catch it. Good luck. =3

Here's a tip for catching any moving Pokemon.

Walk around aimlessly, constantly checking your map. If they come near you, go to their location.

This is what works for me (doesn't use Fly anyway)

Ah roaming Pokémon, yes. They can be little blighters to capture. My best hint for them is always carry quick balls.
The Quick ball work best on the first five turns in battle, giving a very nice chance multiplier of 5.

Even if you weaken said roaming Pokémon, then it runs. Once you see it again, it'll count as the first turn of said battle. So like I said, the quick ball is the best way to go, even though, in my opinion, it's not a very pretty Pokéball...

Also, here is another Glitch, this time on Pokémon Gold, Silver & Crystal.
It is called the Egg glitch, & it is the only way you can catch Celebi without a GS ball in said game...it's a long one, though:

"Celebi egg glitch" - Pokémon Gold, Silver or Crystal
To start, the player must have these requirements:
An Egg - The player must get an egg that will know Beat Up as its third move upon hatching. This can be done by training a male and a female Sneasel up to level 57. Both Sneasel need to have the same four moves in the same order, with Beat Up at position three. Because both parents have the same moveset, the offspring will share this moveset.
A bad clone - The player must get a bad clone, a clone with a name made of only question marks (or, in some cases, blank), its level will be 0 and its gender will be different than the prototype's. If that Pokémon doesn't fulfill all those requirements, then it isn't a "bad clone" and it cannot be used for the glitch. This can be done by Pokémon cloning. The box that is used for cloning must not be totally filled up at any time (otherwise the box is unable to produce bad clones). Getting a bad clone is a case of luck and occurs sometimes during the act of cloning. The chance of getting a bad clone is at maximum if the box has 15 to 18 Pokémon. Another way to increase the player's chances is by turning the game off after he/she selects YES, before any of the "SAVING... DON'T TURN OFF THE POWER." text appears.
Last requirement - The player must get any five Pokémon (preferably weak, common or otherwise useless Pokémon). The easiest way to get them is to clone a Pokémon until there is a bad clone. Because getting a bad clone takes some tries, he or she will get five unimportant Pokémon as a positive side effect of the bad clone action.
Steps
Step 1 - The player must first deposit the egg into a box. The five other Pokémon and the bad clone have to be in the party.
Step 2 - The player must then go to the daycare and ask the man to raise the bad clone. He or she should then take it back from him, and it will be a ?????.
Step 3 - Next, the player must put ????? at the top of his or her party and go to the PC.
Step 4 - Using the "Move Pokémon without mail" option, the player must move any Pokémon (preferably weak or common Pokémon) from a PC box to the top of their party. Its name should become the name of its original Trainer. The player will now have 7 Pokémon in his or her party.
Step 5 - Next, the player must go back to the Daycare man and ask him to raise the ?????. Afterward, he or she should return to the PC.
Step 6 - Back at the PC, the player must deposit the first and second party Pokémon.
Step 7 - Using the "Move Pokémon without mail" option, the player must move the egg from its PC box to the top of his or her party.
Step 8 - Next, the player needs to deposit the next four party Pokémon, but not the egg.
Step 9 - Subsequently, the player should withdraw any Pokémon that he or she didn't use for the glitch.
Step 10 - Having completed the set up, the player should walk or bike around until the egg hatches into a Celebi. However, the Celebi is still unplayable.
Step 11- The Celebi will hatch at level 0. Due to how the game handles Pokémon that level up in battle, the player must allow the daycare couple to raise the Celebi to at least level 2. After this, it works like any normal Celebi.

If you're wondering what a "Bad clone" is, I have set-up a "Glitch centre" thread, for all your glitchy curiosity.

[Disclaimer; This information was C+P'd from Bulbapedia. Though I did have prior knowledge of said glitch, my memory is hazy, as such I had no option but to C+P it.]

OP updated