I collect and mod Maisto 1/18 scale cars...even done a couple engine swaps and stuff...very fun and you can show off your work.
I go to Exoticar.com (but I buy my Maisto cars from Target, KB Toys, Spencer's gifts, Toys R Us, ect...you can get them for as cheap as $12, most cost around $17), you can find good stuff there. Maisto makes nice 1/24 scale cars too(they make everything, motorcycles, 1/12 scale, pull backs, hot wheel size, all that)and you can find those at the places I listed, most cost around $9, some as cheap at $6. Maisto 1/24 scale have opening doors, hoods, trunks, turning wheels with turning steering wheel, they are great. Diecast ofcourse. And they are pre-assembled(I have found several that you have to put together, along with 1/18 scale put together ones(1/18s have 2 sets of rims and other options, although they are harder to find and the ones I've found are either cars I already have or ones that I don't want at all))