I'm currently rereading Yvonne Connell's "Fear of Discovery Series" which features quite a bit of alt-Clark, particularly later on. This series is on the archive, though, so I don't know if that really helps if you are looking for harder to find fics.
Edit: One way you could search for alt-Clark stories is to open up Google and enter site:lcfanfic.com alt-Clark in the search bar. You could also do a similar search on lcficmbs.com to search the message boards.
