This is the 800 lb gorilla of bike parts inventory. If you really want to be able to fix most broken spoke walk-ins then you pretty much need spokes that cover the 250-306mm range. You need them in 2mm increments. Then you need them in 14ga straight gauge, 15g straight, and in double butted. Oh, yeh, and then you need them all - again - in black.
There is no easy way around this. Best bet is to have the customer drop the wheel off, order the spokes, and build your inventory as you go. Of course, that puts the customer off his/her bike for about a week, on the average. That's what I would do if I owned my own shop.
One thing to be very careful about ... If you have to replace a spoke that's some really weird size, get the customer to commit to buying the lot. Give him a break on the price and nicely let him know that the remainder of his 380mm quadruple butted spokes will be anchoring the end of your work bench for eternity, and that's why he gets to buy whatever the smallest quantity you can get for him.
A nice source for small quantities of spokes, at reasonable prices, is cambriabike.com.