The 256kb x 4bit DRAMs are easy enough to find as new old stock on Ebay, search for TC514256AP. The speed doesn't matter, i.e. you can use -70 or -80 parts. Make sure the package type is DIP20, not ZIP.
You can populate the other 4 optional ICs for a full 1MB on board if you don't already have a 512kB expansion module, though you'll need to move a couple of links on the board to make this work.
