ebay is probably a good way to get a CPU.
Newegg.com has procs for cheap when they're on special. Or you could buy one off the forum member when you have 75 reps.
. They offer many retailers with good price. Just make sure to check the retailer rating before you go ahead and pay for anything.
EDIT: Froogle.com might do you fine, too.
EDIT2: Check the second link in my signature. The one by ItsLasher. It might help.
EDIT3: Since you need a P4 S478, consider getting this
. A GREAT overclocker. Same with 2.4C.