Hi vettehigh,
Post #4 here includes a list of some of the better topper sources I’m aware of but adding a topper will change the feel and performance of your mattress in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) so it may not be the best option if all you want to do is add some height rather than change the feel of your mattress. It may be worthwhile considering adding some very firm polyfoam underneath your mattress instead of a topper so that it will have less effect on your mattress.
Other options that may also be worth considering include thinner sheets or sheet straps/suspenders that can keep your sheets tight (see post #3 here).
Phoenix