They used to come up every few weeks on ebay & Amibay, around $100 bucks right up until a year or two ago. Now they're not so common. Sometimes it's worth it to just buy a used 2000 or 500 that already has one installed, even if it's a bit more $$ think of it as then you'll have a whole other spare parts machine, too.
I think the ACA 500 has some hack installed that allows it to expand your Chip memory, but don't quote me on this. Obviously it'll be years, if ever, before the ACA 500 technology ever makes it to the 2000.
There's a few very sophisticated hacks that involve piggybacking additional memory on top of the existing 1MB of motherboard chip mem. Saw a diagram once, wouldn't be something I'd want to try, myself.
