This would be the most logical place to start?
They are (or should I say were) great speakers but I do not think you will be able to get drivers for them, unless you get lucky on ebay. Reference 3A has had a complicated past. The speakers were designed by Daniel Duhay or Dehay or something like that and were made in France and/or Switzerland in the 80s/90s before the brand was sold/moved to Canada. You might also find some information under Innovac from Switzerland. The magazine/-zine UHF from Canada has information. The Master Control monitor was also sold in combination with an iso-baric loaded passive subwoofer as an early two piece bass cabinet/monitor top speaker.
There is someone over here in the Netherlands selling a pair of woofers for (some) Reference 3A speakers, but, mainly due to the complicated history and line up of the models before the current series, it's hard to determine if these would fit the cabinets. FWIW, it's the woofer with the squarish surround of the basket, while the cones are made from some yellow material in a woven structure. If you are interested in the woofers, or info on that matter, I could try to contact the seller.
If your not fussy about identical Ref 3a drivers, you may be able to pick up suitable replacements from company's such as Parts Express or Crutchfield. If the cabinets are finished in a good veneer or lacquer, I would certainly look into purchasing OEM drivers directly from Ref 3a. They (Ref 3a. ) has stopped offering (from what I have seen on their website) the nice finishes and are replaced by an industrial looking grey Nextel. Perhaps good for defraction, but (IMHO) ugly as a knu's hoof. Owners of the Ref 3a speakers with the premium finishes should consider themselves fortunate. They had just started to offer the DeCapo in the high gloss for the veneer finish... Just stunning to look at.
Good luck... .