Thanks so much for the pointers!
I've looked through the marketplace and Reign have two pairs of very nicely shaped thigh boots (the Indios and the Laras), but unfortunately the leather is a bit more muted and dull than I'd like (they do look great tho - just wish they were available in a PVC finish). KC have a fantastic pair of knee high boots (the Munich boots) - great shape, value and variety of colors, but no thigh-high option. Their thigh-boots all seem to be platforms. I will have a look in both in-world stores just in case there are different ones available.
Thanks again for your suggestions!