First off, in todays terminology it's more frequent to use the number of gears at the back as the determining factor. You're looking for 5-speed parts, or possibly 3x5. And I don't think I've seen anything new with 5 speed for years. 6-speed is still around on department store bikes and such though.
However, assuming it's friction shifted a FD isn't particularly finicky about where it's used.
In theory derailer cages should become narrower as the number of speeds go up, but I'm far from certain to which extent that change has been implemented.
My guess is that anything listed as 7-speed compatible should work fine. This automatically puts you at the low end of the range, so price shouldn't be an issue.
Something like a Shimano non-group component should do, or maybe an Acera/Tourney.
Oh, and try to match tooth count span and Seat tube diameter too. A wider clamp can be successfully shimmed to a smaller size(as long as you don't use tape, pieces of inner tube or something equally squishy) but stretching a too small one will most likely be more of a headache.
Otherwise if you want to keep it in "period" style there is always ebay.
