Lowes has a an entire aisle of specialty hardware in drawers and bins as you walk in the door. There is a panel somewhere along that aisle with bolts and nuts mounted in a panel that you can try for size. If you can't find it, go to the bins with bolts. Find a bolt that fits the wing nut. Then buy that size nut. Simple huh?
This may sound paranoid, but I bring a magnet (usually from the tool dept.) with me when I buy stainless at HD or lowes. I always double check that the stainless in the bins is actually stainless. Its not that I don't trust the employees, but when you are stocking 20,000 little baggies of nuts and bolts, its not very hard to put one in the wrong place.