Why wouldn't they be? Is there something unique or different from a mil-spec 1911? Post some pictures.
The majority of 1911s have standard compatible grips with some notable exceptions being Taurus 1911s with ambidextrous safeties due to how the safety is attached to the frame and some Para Ordnance 1911s. I'm sure there are more.
As for good quality grips at good prices, check these folks out. All 4 of my 1911s sport grips from them.