Anyone know, most i look up are in water and i would like to try and get mine as early as possible!
The earliest you can get is at the Oreburgh Gate, which is actually a cave in front of Oreburgh City. Oreburgh City has the first gym. Unfortunately, Psyduck is in the basement of the cave which can only be accessed if you have rock smash, which you get after you beat the Oreburgh gym, so you first have to beat the first gym and then go to the basement to catch Psyduck. Oh, and lucky you, you only have to walk to find a Psyduck! No surfing or fishing, just walking. ~atode!~ ('later!' In Japanese.) Source- http://pokemondb.net/location/sinnoh-oreburgh-gate
In Diamond you can find it at,
Route 203, 204, 205, 208, 209, 210, 214, Celestic Town, Eterna City, Great Marsh, Lake Acuity, Lake Valor, Lake Verity, Oreburgh Gate, Ravaged Path, And Twinleaf Town.