I also have a 15-91. I have purchased parts, including more than one type of quilting foot, from Sew Classic. Check to her website at
www.sewclassic.com. Prices and shipping costs are very reasonable.
Do you have a manual? You can download it from the Singer website free. Since you're controlling the stitch length by your own hands and not relying on the feed dogs, it really doesn't matter what stitch length you select.
I too am just getting into using my machine for fmq. I have tried thread painting and am succeeding by using a light weight bobbin thread and an embroidery hoop.
Let's keep practicing and maybe we'll meet up at Carnegie Hall....
Cheryl