You need to get the length right within +/- 1mm or you will be sorry. NDS rear spokes may be approximately the same length as front, if hub diameters are the same. But not likely with LF hubs.
There was a thread on spokes in the past month or so discussing best pricing for spokes. I don't have a link handy but the best option, which I used, was something like wheelbuildingparts.com (
ahh, here it is). As low as $0.28 per spoke, less nipples. Normally, nipples can be re-used, though there may be some attrition, so if you don't have any spares I suggest getting a few or even a full set of nipples with the spokes.