A pal of mine was just willed a featherweight and is interested in finding a buttonholer, so of course I came here to talk with featherweight enthusiasts...
Anyone have ideas?

Any of the low straight shank buttonholers will work with the featherweights, but look for one in the square green box for a bit of authenticity...that's what was with my mother's machine, and what came with mine... :)

Low shank Singer buttonholers are often available and usually sell at my second hand store for $5. The dark green rectangular box has the vintage black buttonholer and the lighter green oval case has the newer biege model buttonholer. The red rectangular box and the rose oval case are buttonholers for high shank slant machines (Singer 301, Touch and Sew etc). The standard buttonholer came with five buttonhole cams and additional ones could be purchased. We have a dealer who has been in business for years and has a big box of cams that he sells for .50 each.

I got mine on ebay.