Given how widespread the problem of fake chips is (I've been burned myself several times) I recommend buying them from a reputable seller. It is not the cheapest option, but by far the safest.

For instance:
https://www.jameco.com/z/M5M44256BP-8-Mitsubi … de_2288063.html

This is another verified seller: https://www.arcadecomponents.com/memory.html

While the number of the chip nicht vary from manufacturer to manufacturer, these chips are very common. Any fast page mode 256k x 4 DRAM chips in 20-pin DIP case will do, these chips are generally interchangable. I guess four similar chips, possibly as SMD chips, are already present in close vicinity of the sockets. They should have a speed indication (-6 or -60 for 60ns access time, -7 or -70 for 70 ns access time). The extra chips should be at least as fast as chips already present. If -7 chips are on board, both 70ns and 60ns chips will work. If you have -6 chips already installed, you should not add -7 chips.

That's great, thanks.

I assume these chips aren't still in production?

4 of them is 1MB, so it's going to be 1MB to 2MB - the difference:
1MB tops out at 640x480 truecolour, 800x600 64k colour, 1024x768 256 colour
2Mb ups the last two to truecolour and 64k

Matth79 wrote on 2022-01-25, 22:18:

4 of them is 1MB, so it's going to be 1MB to 2MB - the difference:
1MB tops out at 640x480 truecolour, 800x600 64k colour, 1024x768 256 colour
2Mb ups the last two to truecolour and 64k

Nope, 4 of them is 512KB. One of these chips only has half a byte (4 bits) on 256K addresses, so you need two of them for 256 KB. So the upgrade is from 512KB to 1MB, which yields a lot of usable modes, e.g. 1024x768 256 color.