I have a question: I got an Peavey 6505+ 112 and I need a footswitch for it and I wanted to know if I only can use the original one for the 6505 or is it possible that I could buy a third party switch?
I'm not sure. Depends on the function of the amp. Is it just to change channels, or reverb/boost etc aswel? Does it have a normal jack or a serial style input? Peavey will probally charge loads for an official one, but you could try ebay etc to get a used one.

If all else fails drop James at Bright Onion Pedals an email and I'm sure he could build you one. He's a great guy and he built me a customn footswitch for my Traynor at a reasonable price.