8520's can still be gotten NOS from Analogic and probably a few other places but there are also a couple of projects in progress at the moment to replace the CIA's
Lots of CIAs out there are resold broken. The FPGAs are the best option since they wouldn’t break all over again… saw those 2 projects but one is incomplete and the other is not really “fully” tested. There’s that page since a while too but no updates. https://1nt3r.net/j-cia/
Since these were made probably with 100nm+ process, Cost of making them shouldn’t be an issue vs. potential sales, (maybe after the chip shortage ease) since whoever owns the amiga patents are not doing anything better than the last multiple owners since commodore… my money would be on fpgas even if denatures the machine a bit.
https://github.com/niklasekstrom/cia-verilog
https://github.com/bwack/VHDL6526 (6526 and 8520 differ only in their TOD clocks so it should be simple for this to support both kinds)