Places to buy Transformers:

www.amazon.com. Has pretty good prices on most Transformers.

Yard sales/ Flea Markets. Most of the time, the people have no idea what the Transformers are worth. Ex: I bought a ROTF Rampage for 50 cents.

Last day of BotCon. Sometimes people need to get rid of their TFs, so they mark the price down. Ex: I bought An Animated Cybertron Optimus Prime for $4.