Have you tried here?
http://developer.berlios.de/project/...?group_id=3335

There you have (supposedly) ROMs for S(X) and G(X), various revisions.

I remember many years ago when we had to extract the ROM ourselves. I think that since a couple of years the ROMs are freely distributable, but I've been off of the on-line HP48 world for a few years already. (I still use my - real - 48S regularly).