it may be slightly out lf round(bent) it doesn't take much to ding 'em. that pressure on ring is a clue.
If you're lucky it may just need cla.
ebay for the cap, there's an article in"restoring classic & collectible cameras" by thamoa tomosy on building custom lens caps from scratch.
I've made several for old lenses. it takes a little time. but less time than searching for the original.