WOW! I was just going to post a question like this for myself. I took a chance and ordered from
amazon a set of Hadley Bully Air Horns for $319.! I can't wait to get my hands on them. It's a complete setup that includes a compressor, air storage tank, w/ selenoid and switches, and an extra airhose to fill up tires, matresses, etc.. It's very loud with a rating of 133 db @125 psi that can be heard for over a mile. Here is a link if anyone would like to check it out.
Amazon.com: Hadley Bully Electric Dual Air Horn Kit: Automotive
I also have a '05 Access Cab. Does anyone have any installation experience here? Look out lame drivers, you're going to be changing your shorts! =)