I bought 4 4MB SIMMs on e-bay that worked in my Mac II (yay me), so I am up to 20MB now. This was a pull from a Mac II, and I do not know if the seller has access to any more. You might ask him though:

http://stores.ebay.com/macmemoriesstore/

Note that he seems knowledgeable and, at least, knows where the memory came from. This is how I learned that the 4MB SIMMs were from a Mac II. The 2MB SIMMS that he was selling at the same time were a pull from a Mac IIsi. I see that he has more 2MB memory for sale at the moment (but no 4MB? - I didn't look very closely), but don't know if anything would be suitable for a Mac IIx (or if any 2MB memory works in a Mac IIx).