Check SM's uploads section. You can also check sites like 3D Cafe and turbosquid. It's easier to find OBJs than it is to find .ma, .mb, and OBJ files can be import into Maya if you activate the plugin.
Of course, you COULD also make them yourself...
AIM: mhcannonDMC
"If you love your job, you'll never work another day in your life."