Cattle Visions carries one of the largest selections of Shorthorns out there.
Several of the other semen distributors - Bovine Elite, Universal Semen Sales, etc. have a fairly standard set of Shorthorn bulls available.
The latest ASA AI sire directory just arrived in the mail last week - go to the ASA site, I imagine there's somewhere on there where you can request a copy.
Sexed semen not available on a lot of Shorthorns, that I'm aware of, and none that I've seen on any bull that I'd consider using.