Yes, they're replaceable. You might check to see if Meprolight makes a site. A place called Arizona Gun Runners sells them for the best prices I have seen. It's a very popular gun now so there should be lot's of aftermarket goodies.
There was also a web forum designated for XD's/HS2000's only, you might do a google search...I used to hang out there a little some time ago and I forget the web address!
Someone there would know what's available.
Also, Dawson Precision might be an outlet now too. I bought my Fiber Optic Sights there for my Glock....they are AWESOME but not any better for night shooting if you're using it as a CCW pistol.