It's supposed to be in the Great Marsh, if you mean D/P/P. I hate those areas. It seems like an impossible task to catch a rare Pokémon there; give me a Quick Ball and False Swipe and I'll wear it down for another ball if need be.
I don't need a Celebi, I have the Lv.50 event one that has since met its friend Zorua at Castelia City.
I'm still looking up Carnivine, but still not liking the prospect. I can't get it until I get to the Viridian Forest, though it appears in the Ilex Forest as well with Sinnoh Sound after the E4. No worries, I'll get one for ya, it just might take some time.
Unfortunately, Latios is still some ways away; it seems I have to visit Copycat and get a doll first, then upon leaving the city, the roaming Latios is triggered. I'm only 4/8 on Gyms, and obviously, I have to finish the Pokémon League to continue exploring Kanto, sooo...
I'm going to have to rely on the Action Replay someday to grant myself an Enigma Stone in order to catch Latias at Pewter City. The 3DS version should arrive. Someday. That will let me unlock the Lock Capsule item as well, getting me TM95, Snarl.