Is there any place that does affordable thigh-height boots in the bigger sizes?
Ebay is very limited on what they offer.

Trying to find size 11 boots for my Drocell cosplay and I do wonder if I can do any better than the boots that I currently use for 3 of my cosplays.

I dont know anywhere else. But how about making your own? Not as hard as it imediately sounds. Start off with a donor pair of shoes that look right and then make the leg parts like sleeves for the legs - I am working on Blackadder thigh boots and this is how they look on close inspection. You could fasten the bottom under he arch of the foot/shoe with black elastic stirrup. Leather look fabrics are availabe. Could be a fairly simple project and a lot cheaper.

Where is a good place to get the material for the boot sleeves?