I just take the wheels out of other cars. Each car's only about a buck and I've found that the goofy fictitious cars (hot rod toilet, animal cars) have the best motors and pipes or spoilers. I'll buy one of 'em just for a scoop or a blower engine.
Each axle is held in with a kind of tension prong thing, and you just pry it out and put it in the frame you're building.
I start out drilling the rivets. Begin with a small drill bit and work up (it's a metal rivet surrounded by plastic in most cases, and can get ugly quick.
From here all the interior, glass and body pull apart and you can paint the interior and so on while you do the next step:
Then I dump the body in a jar of lacquer thinner and let is sit over night.
Hit that with a wire wheel or brush, then knock yerself out with paint and customizing.
