5 cogs in the back? You're almost certainly using friction shifting, which means pretty much anything will work. Well, so long as it's a braze-on derailleur in this particular case. Derailleurs made for fewer speeds may have slightly wider cages to accommodate chains made for less cogs (and making for less chain rub when cross-chaining), so I'd personally try to shoot for 8-speed or lower.

Here are a few new derailleurs that'll work for you:
Shimano Tiagra FD-R440 on eBay, $14.60 with free shipping from Taiwan
Shimano FD-2300 (Claris) on eBay, $11.97 with free shipping from Minnesota
Shimano Claris 2400 on Amazon, $18.27 with free shipping

eBay has plenty of used ones in varying condition. I'm a little hesitant to suggest one in case I happen to pick a lemon or a bad seller, but there are LOTS you can evaluate yourself and choose from. There are a few braze-on Shimano 105 FD-1050 or FD-1055 derailleurs which would be a great fit. Shimano 600. A few SunTour models.