How can I determine the latest released firmware for my Samsung device?
In general there really doesn't seem to be a good way to keep up with updates that haven't been released directly to you.  I find the best way is simply to be involved in the "Android ecosystem" &mdash; reading Android blogs and news sites, and so on.

Sometimes there are stock ROM lists maintained on sites like XDA, for example this one for the i9001, and you can monitor those.

For actually acquiring ROMs, see: Where can I find stock or custom ROMs for my Android device?