All I have seen for free is monsters invade oz and monster life ? Are there any other good ones out there ? Maybe someone should make a "list of Nintendo-inspired games " thread
Beastie Bay made by Kairosoft is probably one of the better ones you'll find. Some people would also recommend you Dragon Island Blue (/Hunter Island), but I personally didn't like it at all.
There are no substitutes that will come close. I tried the same thing - ended up breaking down and buying a DS for $50 and Black 2 for $17. It was well worth it.