[DonRotolo]

I have a quite small mill, and it uses 3/8" (5/16-18) T-Nuts. 1/4-20 would be a mini-mill I suppose.

For 80/20 type stuff, I have used carriage bolts to good effect. Cheap too.

For a mill, my recommendation is to make them. Get some long steel bar, mill in the slots, drill and tap the holes, and cut it into nuts. Maybe not cost-effective at $60/hour, but student labor is cheaper...

Note: Don't tap the holes all the way thru, to keep your studs from marring the bottom of the slot or, even worse, breaking the slot open.