I have the HQ Sweet Sixteen (same machine). HQ markets HandiGadgets. The key is that the ruler is 1/4 inch thick. A ruler meant for use with a rotary cutter is too narrow and you risk hitting it and knocking your machine timing out. I have the HQ half circles, HQ Minicircles - a neat ruler with cutouts for little circles that you can use on the inside and full circles, and a straight edge. The Line Tamer is another ruler I've heard excellent reviews on by SS & Tiara owners - for straight line quilting.
Quilter's Rule makes a wide ranging set of templates for use on longarm/midarm machines.
Cheers, K