Try KEH. www.keh.com They have an SC-1 (the standard screen) in EX condition right now. Their computer auto-complete converts SC-1 to Soft Cover-1, which can be confusing, but if you know that, you're OK.
Screens from the LX will fit just fine and are less grainy and brighter. Their extra brightness will probably throw off the meter slightly though, which can compensated for on the ASA dial or by getting the meter adjusted. LX screens have similar numbering schemes to MX screens. For example, an LX's SC-21 is the same type as an MX SC-1.