In UK, there's a couple of places on <a certain major UK auction site> that do essix retainers... about 40 quid each, 75 quid for a pair. make sure to select one that sends an impression kit. There's one listing I saw for only 25 quid, but you need to supply your own impressions
(search for "Essix Retainers")
I have some: nice & thick (1mm I think). Need to get another upper: had a crown fitted, and had to cut off part of the retainer to fit!!