I am pretty sure that Jack Stephenson was the first to sell them.
There is also Kooka Bay.
That is just a guy (Bender) that makes them on order.
You can specify the exact size you want and the amount of fill.
His customers at BPL (Backpackinglight) seem happy with the product. Apparently his down mat is a bit slow to deflate but he has added an extra purge valve now, so should be OK.
Here is on of his comments :
"FYI the 70" Exped Downmat 7 has 6oz of 700 FP down and weighs 31.7oz. The 72x20" mat with 6oz down I made for Bradford weighs a hair over 20oz"
Contact Bender athttp://www.kookabay.com/