I didn't see any good premade kits of mosfets from the usual sources either. Assuming you meant MOSFET. Most assortments seem to have mostly normal transistors.
Your local electronics store? Get few different kinds, first specify how much current you need and so on, you probably want a logic level fet if you want to control it straight with arduino pin. (with resistor between to reduce ringing) If you just want to whack something in there, then disassembling few products might be an option. Old computer power supplies might be a good bet for some beefy mosfets. Last psu I took apart had two irfb4110, I could have used more for free. The psu had most of it's low voltage electrolytics fail. Your typical psu also has it's mosfets already on a heatsink, properly mounted, thats a whoppy saving right there which even the chinese cannot beat easily.