There's afew:

www.spinlitecycling.com, great place and while they only have a small selection of Velocity rims avaliable, they're usually in stock at the shop and ready to ship within a day or two. I recently ordered a built wheelset from them and Lyle Strickland is great to deal with.

www.speedgoat.com also has a great reputation here on BF and has an enormous selection of rims from every manufacturer you can imagine, but i'm pretty sure they don't stock everything listed and will have to order it from their catalogue which could mean an extra week or two of waiting depending on what you get. Prices do seem a bit less expensive and they might have free shipping on your order.