Are you thinking about backstage (dressing room) paging capability, as well as front of house announcements?
They are usually 2 different systems, in the theatres I've been in, and can vary widely. Backstage systems usually have speakers in each dressing room and the hallway areas, with a mic of some sort at the SM console. Sometimes this is built into a com system (like Clear Com or Telex), so you use your headset mic to make the pages. (Somebody makes wireless belt packs with a paging button...although I've only seen that once). Sometimes it's a seperate mic. If you have speakers existing for program feed, I would bet tying into that isn't too tough, but if you don't have any infrastructure, it will need alot of work.
For front of house (like cell phone, or emergency announcements) I have usually had a stand alone mic that is run to the regular house speakers.
I can't tell you specifically about suppliers, but your local lighting/sound/theatrical supply company can probably help you with equipment pricing.