MultiTech has discontinued the only USB modem that works with VSI-FAX, but the working MT5634ZBA-USB model is still available elsewhere. We suggest starting with an eBay search, which has worked well for a number of customers seeking replacement modems in a pinch. All of the typical eBay caveats apply (i.e. purchase from a reputable seller, make sure there’s a return policy, verify the warranty status, etc.).
Please note that while the model numbers are similar, this is not the same modem as the newer MT5634ZBA-USB-V92 model, which is not certified for use with VSI-FAX on Linux.