French threaded bottom brackets are an odd size, and have RH threading on both sides. Basically, nobody makes them new anymore, except that Phil Wood sells French threaded retainers for their bottom brackets. You have to hunt around on ebay, but stuff shows up there from time to time, or other vintage parts dealers. Velo Orange sells, or used to sell, Edco bottom brackets that are a friction fit that will work with French bottom brackets. Some say you can get the bottom brackets rethreaded, but I would only do that as a last resort.

You may be able to find a spindle that fits into your existing cups with a tapered, non-cottered interface to the cranks. Rivendell sells new tapered spindles, probably others, somebody around here probably knows whether these would work with your existing cups.

Anyhow, a lot of options do exist for this problem.