Wheelbuilder.com gets a lot of favorable posts here - and I can vouch for them as well.
And they do have Sapim double butted in 310 length for $0.98 including brass nipples.
They have a nice selection of DT and Sapim and that's all.
But $1/spoke with nipple is not expensive for good stuff and it's really cheap for über stuff.

Pricing for my last build (DT Competition):


There is a guy that gets recommended in here sometimes that cuts spokes to order - that might be what you need to do. I don't remember his name or site.