This is a good thread for SR9 holster options:
You can also find a list directly from the manufacturer here:
It is easy to find a holster for your pistol that has strap retention (i.e. Galco FLETCH), however I'm not sure how easy it will be to find one that mimics the Serpa. If you don't want leather, you can still get strap retention in a nylon holster (listed in the above).
I've run into the same issue with my Kahr. I have a Serpa for my 1911, but I think I'll have to settle for strap retention for my P9.