I use Sapim spokes. I can get them by the box from a number of sources. Just found a new one being Echelon Bikes in Pershore for £30 for 100. Not mailorder though if you’re not local.
On-One can supply Sapim spokes too. Not sure of the price though (dont go on what their website says for Spaim Race as there’s no way theyre £2 each – thats the price for CX Rays.) Mike Jenner there is a “sapim” wheel builder.
Sapim are available in 291 (I have a box) and I also have some DT Swiss Comps in 271mm so they are available.
This german site works out quite cheaply including delivery
http://www.roseversand.de/output/controller.aspx?cid=156&detail=10&detail2=2313
for DT comps.
No 271 or 291 listed, but since there seems to be some confusion over the ERD of the rim, Id go for the +1mm longer for safety sake. It wont be so long as to be a problem.