What is the most available ammo and then from that, what are my rifle options?
Can't beat .22LR.
Keep in mind that C&R does not necessarily equal milsurp!. There are literally dozens- if not hundreds- of types of 50+ year old vintage .22's out there.
My favorite "shooter" C&R rifle is an early-model Winchester Model 69. I've been considering picking up a Model 75, which is the heavy-barrel competition version of the same basic rifle; it takes the same magazines, so I already have spares.
If you favor military rifles, there are many .22LR trainer versions of "real" milsurp rifles out there, although some are quite rare and consequently may run expensive.