Well, doing a bit of research and going through my files, it appears (I can't confirm for absolutely sure, however) that Direct Sourcing Alliance LLC went out of business over a decade ago.
If this is so, you probably will not find a brand new direct replacement lens/cover for your fixture. You may be able to find a used one in a salvage yard or a used RV parts store somewhere or could even find a pre-owned entire fixture on ebay or similar site.
My suggestion is to bite the bullet and spend the money to replace both fixtures with new LED replacements. LEDs as you know are much more efficient and are suited for RV use where conservation of electricity is important at times.
The most logical replacement brand would be Thin-Lite. They are a big company (as opposed to Direct Sourcing Alliance that was a very small operation) and have lots of options to choose from.
A possible replacement might be the highly-rated LED ceiling mount fixtures from Thin-Lite. They are sold at numerous vendors, are OEM for some RV manufacturers, and parts like the diffuser covers should be readily available should you need them again.
Here's one of equivalent watts through Amazon --
HERE, or one of a little less watts
HERE