When I cold water dive (which is 100% of the time), I pair my 4 mm sailing neoprene boots with a 3 mm neoprene sock. It works like a charm. I don't think the socks are made for diving though... my memory is failing me, but I think I got them at a skiing store, though maybe a big sports chair might carry that kind of thing.
The sailing boots (made for dingy sailors) might help with the half sizes issue. They do tend to be thinner (thus the sock), but built more rugged than most dive booties. And they are especially great for those of use who have small feet and therefore have problems with the top of the fins' foot pocket digging into the top of our feet since they have a reinforced panel there for hanging out of dingy's by one's feet.
Hope this helps.