Hi Andy!
I am in the same situation as you, and have a 36t Surly chainring. I have corresponded with Surly and it appears they won't be adding stainless chainrings larger than 36t in four-arm, 104mm BCD at this time.
Rohloff's FAQ page cites several chainring sources, but I don't know if they are produced in stainless thin enough for a Chainglider:
4-arm chainrings for Shimano XT/LX cranks
Mountain Goat Lonely Wolf Kettenblatt
4 arm chainrings for the 2001 Shimano XT/LX cranks as well as chainrings for nearly every conceivable crank and mounting hole diameter can be produced by various specialist firms such as Speciallites TA (France).
Suitable chainrings are available in both black or silver finishes with a choice of 36-38-40-42-44-46 and 48 teeth from
http://www.tnc-hamburg.de
Mountain Goat Lonely Wolf
Bolt circle 64/104 mm
Teeth 39/42
http://www.mountain-goat.de
NC DH-Rings
http://www.nc-17.de
Idworx offer some larger stainless 'rings in 35t, 39t, and 43t. They advocate odd-odd chainring/sprocket combinations, citing longer wear. I'm guessing their 43t 'ring will result in a less than ideal mating with a Chainglider.
The Idworx parts store is here:
http://www.idworx-bikes.de/de/informationen/parts.phpAlthough Idworx is a competitor to Thorn in bikes, I offer their parts link as an option simply because stainless 'rings in larger sizes are so difficult to find elsewhere. I am not sure how thick the actually are, and cannot confirm they will work in a Chainglider without creating undue drag. I presume they are thin, but you should confirm via correspondence before ordering. GoogleTranslate is your friend here, as the site is only available in German and Dutch.
Best,
Dan.