I don't know if it applies to Featherweights but Singer used to show the part numbers of the attachments that came with their machines in their manuals. If you have the right year manual for your machine you might be able to track down the part numbers for your set that way.
If you're just after something that fits then any low shank straight stitch attachments will work. I think most Ebay sellers put "Featherweight" in their ads hoping to either sell faster or for more $ or both.
Rodney