At Trucks at Carlisle this past year, there was a company in Maryland that sells airhorns and install kits for trucks.
Was going to order one for my jeep, but found a used horn for cheap at the show instead.
They have many horns for sale, long short, round end, rectangle ends, compressors, air tanks, lots of stuff.
The have a website at http://www.ricksontruck.com/
, but not much info yet, just contact info for them.
I was going to get the horns model 1609, black (powder-coated i think). About 15" long for the long of the two horns.
install kit was #1730.
think they may have a 3 horn train setup also. If not, some company in NJ has a four train horn kit that is awesome sounding, like railroad salvage supply company, or something like that.
Prices have gone up from the maker, so not sure what they cost now, but they had bee about $36 and $29 repectively.
the install kit was so you can use a button, instead of a pull cord.
not sure what the compressor they sell are like, may just be for the airhorns, so may not be too suitable for airing up after a ride, but maybe they are.
Air tanks they had are kind neat, not the normal cylinder like Sun tanks, but more like a pancake compressor shape. Larger diameter circle, but not real high. (see attachment for shape)
The also sell a really cool conversion kit for full size trucks, so you can use 19.5 inch wheels and tires on your truck. The tires are actually semi tires I think.