I did some research some time ago.
I have lost my collected data, but I can tell you that they are very hard to get.
There exists something where you can put in normal SIMM memory, it's called the SIMMConn,
but the website is really really old, and the product isn't available anymore i think.
http://simmconn.tripod.comYou can put a SIMM on there as big as you like, but the memory can never be used beyond 28MB
The original upgrade boards have part number CT1950 for the 24 MB version.
This will give you 28MB in total.
Some russian site about it.
I'm looking for one myself, an you best bet is ebay, craigslist and similar sites.