I think the best place to find extensions other than those in the Public Library is now the Friends of Inform Github repository. (Extensions that are updated for 6L02 should work in 6L38, I think.) Downloading is a bit of a puzzle–when you get to the page where you’re looking at the extension’s source code, right-click on “Raw” and choose “Save as…”.
You might also want to look through this thread for a discussion of which old extensions are compatible with 6L.