The 15-91 is a low shank machine. Any zigzag adapter that fits a low shank would work.
Here's one on Ebay right now. No affiliation with seller, just one of the first ones I saw. Singer also made a version with removable cams for different stitches.
http://www.ebay.com/itm/Vintage-Sing...item3ceaf326a8
I've seen various reports of success and non-success using them. I have a Greist made one but haven't played with it much.
Rodney