You can catch gastlys in all of the following areas:
Sprout Tower
Burnt Tower
Union Cave
Dark Cave
Hope this helps
The first place you can find Ghastly is in the Sprout Tower located in Violet City.
use gastley as a ghost Pokemon and us its poke assist
go to the sprout tower in the town where you get the first badge at night.
Assuming you mean the Pokemon Gastly and not some other Pokemon unknown to me (of which there are few), you can find it in the Old Chateau near the exit to Eterna Forset. You will find that it is the only Pokemon in there, other than Rotom, which I suggest you don't bother getting coz it's stupid.
You can't find Mew in Pokemon Silver.
You can get Gastly only by trading from Fire Red/Leaf Green it's in Lavender tower.
there is not a secretpotion in Pokemon silver Daniel robertson
You find Moltres in Mt. Silver.
gastley evolve's at level 25 trade hunter to get gangare
You don't find it. You find larvitar, which is located at Mt. Silver.
to find out how to catch teddiursa in Pokemon silver... YOU CAN'T IDIOT
You can't find Mew in Pokémon Silver.