I'm an IFR rookie, but I just happened to have read a bit about RAIM last
night.

If I can speak on behalf of Barry, he's asking for RAIM _predictions_ for a
certain route (or the whole country). RAIM availability is not a constant;
you can't get it from a simulator because it's subject to real-world
variables. You can't get RAIM forecasts from a receiver, right? The
receiver can only confirm integrity "right now". Later on, the required
five satellites may not be available due to things like scheduled service
outages and atmospheric conditions. Wouldn't want to find that out at
arrival time.

To quote Jeppesen:

"In practice, the easiest and most reliable way to ensure that you will have
RAIM for the approach at your ETA is to specifically request GPS RAIM
availability from your FSS weather briefer."

Still, I would echo the question: where do the briefers get this info?

-Scott