Not following the cost of a box of spokes as being an issue, the spoke needed is ordered individually from Madison, the consumer site shows this, the LBS would need to check the B2B to confirm stock availability. Would not expect a LBS to carry this as a stock item though. (link is for a front spoke but the process the same for a rear)
Would personally be unhappy with a LBS for not fitting the correct part, if the part was available, as they should have been able to tell you when you dropped it of, if it was or not, and given you options at that time for alternatives.