I purchased a bunch from a place called Fastenall. I bought both
phillips and slotted, in several sizes, 1/4, 3/8, and 3/16's (if I
remember correctly). They were really cheap, compared to buying
them a dozen at a time from Walthers. I think it was only a couple
bucks for $100. Having the different sizes helps for different car
mounting situations, and I hate mixing phillips and slotted on a
single car. Oh yea, 2-56 was the size I bought. Kadee makes a
2-56 drill and tap kit (as well as a few other sizes).