Which rims, exactly, do you have? Brand/model name?
Finding the right length spoke is easy, if you have the hub and rim at hand or if you know their given measurements. Once you measure the effective rim diameter, flange diameters, and center-to-left/center-to-right flange measurements, you plug all that into an online spoke calculator. (Many calculators will have the specs for common rims/hubs saved.) choose your spoke lacing pattern/# of spokes (I'd suggest 3cross), it'll give you the exact ideal spoke length.
Also, I just got some 13g spokes and they seem to have a nice balance between being beefy but still fitting in most standard hubs/rims. If your rime are drilled to take HD spokes, that might be a factor in ordering both hubs and spokes.
PS-If you want top save money, find a front hub that will use the same length spokes as the Nexus. there were different flavors of Nexus hubs, but an example would be that the Nexus sg-3c41 will use the same spokes as a Sturmey-Archer HBT front, assuming both are 36h and laced 3cross into identical rims. This will save you some coin....
HTH
