The Flex lenses are the same as those for the rangefinder version, but mounted as pairs as in a TLR. Flex version 90mm lens are relatively easy to find, but I've not seen too many examples of the 58mm, 135mm or 180mm. I don't think the Flex was terribly popular, and it's a brick (just like the M) but the ground glass viewfinder lets you do some different tricks (see https://www.dantestella.com/technical/omegaflex.html).
I've got a Koni-Omega Rapid (rough but working) with 90/3.5 (fair condition) and 180/4.5 (very good condition) that I'm looking to part with.
The camera body is quite beaten up, as it was used for decades as a local newsman's primary camera. I'm the 2nd owner and have used it a little bit with no major issues aside from very occasional gap-variation between frames. I can post some pics later if you want to see more.