I can get them at an RV salvage place near my home. Didn't cost much and they fit the existing rails just fine. But I'm blessed to be living in Indiana where so many RVs are built and there are several salvage outlets around the state.
A countersunk head screw is the name of a screw with cone-shaped sides on the head. I didn't imply that you countersunk (drilled) the drawer guides. The countersunk head screw will sink lower into the hole in the drawer guide and maximize the clearance between the top of the screw head and the drawer roller.