1st off - where are you located? so many systems out there on different bands that one radio can't cover the whole country.
I would start by doing some google searches, and checking with
http://www.trunkedradio.net to see what systems might exist in your area. You can also search on the fcc's site to see what is licensed as what, but you won't get any data that will help you determine what is what.
2nd - What is the radio flashed for? You need to give more information regarding the radio your using. What's the model number? Flash Code (or MFG Code as it's sometimes labled on the label)?
After figuring out some of this, your going to have to get it programmed. Your going to need the software $370 or so, plus a rib and cable(s) $200+. SO, your already the cost of the radio, plus all this other stuff, just to program it.... You'd run into the same problem with the portable.
You also can't walk into a radio shop and say - I want this on a trunking system. The'll laugh at you (most likly).
Do some searching here on the board regarding trunking, RIB, and astro, and you'll be reading for weeks. It'll give you a good overview however of what you might want to know before messing around with the stuff.
It ain't cheap.
Also, read up on your radio model and the programming primer on
http://www.batlabs.com.
Good Luck, and we are here to help. I issue a word of caution however. Be careful how you structure your questions, and I URGE you to reserch them before asking. If you come off sounding like you haven't done that, you will not get very far with anyone here.
Enjoy.
-Alex