Gen 2/3 Moemon (that I have seen in my game) by Route: (Also some other interesting finds)
Route 1: Sentret, Zigzagoon, Poochyena
Route 2: Hoothoot, Poochyena, Sentret (and Dratini)
Route 3: Slugma, Yanma, Cyndaquil
Route 4: Totodile, Chikorita, Cyndaquil, Torchic, Marill
Route 5: Sneasel (and Glameow. Possibly available on other nearby Routes)
Route 6: Taillow, Smeargle (and Lillipup. Also available on other nearby Routes)
Route 7: Taillow
Route 8: Taillow, Murkrow (Also Porygon)
Route 9: Wingull, Murkrow
Route 10: Wobbuffet, Wingull (and Shinx)
Route 11: Absol, Murkrow
Route 12: Seviper
Route 16: Emolga (Possibly also available on other Routes)
Route 23: Ralts, Poochyena
Route 24: Treecko, Marill, Mudkip
Route 25: Torchic, Marill
Pokemon Tower: Misdreavus (Upper floors)
Rock Tunnel: Torkoal, Larvitar
Safari Zone: Miltank, Seviper, Heracross (Probably others as well)
Viridian Forest: Spinarak
Gen 4+ Pokemon do not appear in the Pokedex. The Routes listed for them are where the game says I caught mine.
Edit: I also want to report some bugs I've been seeing with trainer's Moemon using moves they shouldn't be able to use. For example, Koga's Crobat and Seviper use Selfdestruct, but neither Pokemon is capable of learning Selfdestruct.