No luck with the old DRAM from that supplier. I've given them a list of other compatible parts, but I'm not holding out much hope.
The only other option I've found is the Psion Series 5. Turns out they used many of the same parts, including the DRAM! The only issue is that some use slower chips (70ns) than the ones I've found in the 3mx (60ns). It might not matter, but I'll need to do some tests.
I've ordered a dead S5 from eBay. I'll see what's wrong with it, then maybe harvest it for parts.