You can also try Thayer Rags Parts {
http://thayerrags.com/ }. CD is a member here and I've done business with him several times.
"Generally" foot controllers can be cleaned, adjusted and / or repaired. I've done several. They will usually come apart with a couple screws and there are just two wire connections inside. If you are good at searches do one about foot controllers. I've posted pics here within the last couple weeks showing the insides of a couple different vintage foot controllers.
Or if you can't find a vintage one you can try Sew-Classic {
http://www.shop.sew-classic.com/ }. They have both the old style resistance type and the new electronic types. I've bought some of each from them and they work well, even if they are not vintage looking.
Oh yeah, make sure the machine
and motor are clean and properly lubed. Dirty and or gummed up machines and motors will not run properly.
Joe