For various reasons I want to find FCO options for different luminaires. Could I get these from somewhere online, or I have to get a glass store to custom cut?

One reason is that I wish to compare photographically the difference in light distribution between the refractor & flat glass with the same luminaire. Then when I'm done, I'd like to sell the luminaire with both refractor & flat glass.

Some older lights probably never had FCO lens, like:

Westinghouse OV15
Westinghouse OV25
Cooper/Crouse-Hinds OV15
GE M250R
Westnghouse OV-14B (not possible?)

Might have to get them custom cut at this point as I'm not sure the parts are still available from the manufacturers. Most of my FCO luminaires used 1/8" thick glass.

Anyway all the "small" luminaires on your list aside from the OV-14B use the same glass footprint so one should fit all.

As for the Westinghouse OV-25, a Cooper OVF flat glass uses the same footprint and should fit.