When the Macintosh XL was available, Apple used 512K memory boards, of
which two were supported in the system. AST used to manufacture 1MB memory
boards, but they have since discontinued that product.
Sun Remarketing is an excellent source for support on the Macintosh XL.
They also carry outdated third-party products for many discontinued Apple
products. They may have some of those AST board in stock.
On AppleLink, you can find contact information on a company mentioned in
this article by clicking the "Library Index" button in the lower-center of
the Tech Info Library search window, then opening the "Third Party Company
Directory" folder.