I had a faulty wheel once, it broke spokes like nobody's business. During a couple of months I must have taken it to 2 different LBSs for 5-6 repairs total, until finally the latter LBS decided they've had enough and cut me a very good deal on a new wheel. On my request they included a couple of spare spokes, which I promptly duct taped on rear rack tube.
The new wheel took care of the problem, and the few occasional broken spokes I've had after that I've replaced myself. When I'm down to 1 spare, I visit my LBS with it and buy maybe 5-6 identical spokes. I've been fixing both sides (drive side / non drive side) with the one length replacements. Don't know if that's the ideal way to do it, but it seems to work for me. I don't even know what spoke length I'm using, as I've never had to find out. Maybe if/when I start to build my own wheels...
--J