email the online store you are buying from. If you are buying all the parts from them some will guarantee they will come together correctly, I think I ordered from lick bicycle, and when their sales rep screwed up, they sent me a new set of spokes. I built the wheel around a Phil Wood hub so it was a fairly expensive sale. I think Harris cyclery will also guarantee, but for a smallish sum they will also build it for you, which might be worth the money if you value your time and don't need a learning experience.
Otherwise use the parts covered in the databases of hub and rim dimensions. Post you plan on this msg board to see what others think. If you are looking for cheap people who cut generic spokes regularly post on ebay so you can buy from them and if you screw up, buy again from them until you get it right. I have built 3 wheels using spokes off ebay and have not been screwed by any of the sellers, the generic spokes have not failed.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.