Just a small point of contention, but the IRL no longer exists. The sanctioning body is now called INDYCAR (seriously, it's in all caps. I'm not just trying to put emphasis on it), and their top tier series is the Izod IndyCar Series.
Sorry, not trying to make you feel dumb or anything, it's just my favorite series, and thankfully, the Vision is dead.
Their frequencies, at least for the Izod IndyCar Series and the Firestone Indy Lights, are included in all their spotters guides, which are available on their website.
Hopefully, next year, I'll be at Indy for the 500 to see the new chassis/engine combos and hear the whine of the turbos again. I'll have my PSR 500 with me, for certain.