I found a UK supplier for ev50 in low volume - pentonville rubber in London (not as dodgy as it sounds, but not an ideal name on your credit card statement!).
unfortunately, I and they thought it was ev30 I was buying, but the density I've measured means it must be ev50 so it's 150g/Sqm vs the 100g ev30. Not the end of the world, but there's quite a few ev50 suppliers and I'd really prefer the lighter stuff.
I've used goassamer gear thinlight matts for this for years. 3mm evazote foam, pressure blown. Very high quality, robust and light (70g for a 150x50cm piece).
Unfortunately I've found it impossible to get hold of the EV30 evazote in 3mm thickness in the UK without ordering at least £250 worth, and my thinlight matts are getting a bit worn out. So, if about 20 other people want some we can get together...
If you're not completely weight-obsessed, the tent underlay that edh linked to is pretty close, but its higher density and/or thicker.