Bikeville in Philly. Go there. Be courteous and you should get the attention you desire. They probably have what you need NOS, in my experience it will be priced by its rarity and your attitude. Their upstairs has a museum worth of stuff.
I think the freewheel problem is a worn cog or two. Those alloy mechanisms were designed for Special Use, not for everyday. Think time trials only.
That is how we viewed them way back.
A worn cog will show for example by; when it works in the workstand but not under load. Keep track as to what cogs it skips in. Another chance is to check for chain stretch. If you are lucky the chain has stretched and the cogs are okay. If you have a new chain and there are no stiff links, you are back to the freewheel. There are a few members here who have much experience working on freewheels, (I don't have the patience) trouble is that if a spring or pawl is needed it must come from a donor mechanism, there are two pawls, chance of having both fail is low.

Campagnolo was the only maker who made spare internal parts of that type back in the day, as DD advises, terrific units, strong alloyed cogs, you pay the Campagnolo price. Good luck finding spare internal parts though, the guys who bought the service chests keep them for themselves, and quite understandably, they fetch a handsome sum. Cogs do show up from time to time though, rarely for the other brands. If you find one, you need the special puller or you will damage the body, period.

Everest was standard equipment on Masi (both venues) for a long time in the 70's through the 80's. We considered them more consistent than Regina in quality.