Alibaba. You can potentially buy DDR5 RDIMM at a couple hundred bucks a stick. You typically have to buy them with a minimum order of 10 pieces, and many of the sellers seem shady. I recommend going with a seller that has been around for at least a couple years, a return guarantee, and comparing the screenshot of a listing against the specs. If the RAM depicted isn't a dupe of another seller, not blurred, and has the right things on it, then it is probably a legit source.
Also, watch out for shipping fees, and some sellers might cancel orders if they don't reel in a big enough order to justify their bottom line.
Ebay is also an option, with similar caveats.
Here is a POTENTIAL source for RDIMMs, with a minimum order of 5 pieces for $141, +$20 shipping. The pics match up with the listing, though no unique photography is in use. If my Ebay order fails, I might give this a shot.
64GB DDR5-5600 PC5-5600B ECC Registered RDIMM 2Rx4 Smart Memory Module for ProLiant Servers
0000000
I bought 32gb of DDR5 RDIMM for about $600 from Ebay. Mind, it hasn't shipped, so we will see in a week whether the seller is good. Also bought a used Threadripper Pro 7000 for $1500. Again, whether it is valid is a question that will be answered within a month for me.
Assuming things go smoothly for me, my goal for the next year or so is to build up a warchest for the AI bubble popping. My TRX50 AI TOP can accept up to 8 RAM sticks, provided it has a Threadripper Pro. With any luck, I would be able to fill it out for $2,000 or thereabouts when the pop happens.