I need to catch one for my Pokedex, but I can't find one anywhere. Like, can I find one anywhere in the game, or perhaps in the Safari Zone?
Yes, you can through the safari zone.
If you have 8 water items in the desert you can find Lotad.
If you have 12 water items in the Plains you can find Lotad.
(Sorry for the other answer, it didn't have as much information on the Johto Safari zone on the DB page for it)
Although, without the safari zone you have to trade from another SS game or D/P/ Platinum or HG. You must actually meet up with the person you are trading with or you must have another system.
Hope I helped!