You mean "slips"? Never used them. Kind of neat tool, but all of the curves I've layed down always seemed to be some odd ball radius, which I handled with a yardstick and a marking device attached at the required radius.
I've also made radius templates out of masonite hardboard which helped when a center point was out in thin air.
IF I was building a large layout I'd probably invest in a set, but if like you said, they're hard to find, I'd probably stick to my caveman style trammel that I've been using.