I would search yard sales, pawn shops, and thrift stores for anything with a 1 1/8 threadless headset, front suspension, and a front disc brake. You should be able to find something for around $100 that will get you everything you need, with the possible exception of the correct bottom bracket gear. You will probably need to either buy a bottom bracket adapter to use a 3 piece crank or buy a 1 piece crank with an appropriate sprocket and bearing cups. the adapter is the cheaper solution, but isn't quite as strong as a one piece crank