The most common replacements that I've seen over several forums are:
Fiamm Freeway blasters. ~$15/horn ($30 for both high and low). You can do both, or just add the low tone to your stock horn (also made by Fiamm). I wouldn't say these are super loud, however they'll make it far more noticeable, and it will sound like it should have from the factory. You can make a Y-connector with about 8" of wire and several spade connectors and run both horns off of the stock wiring. I installed these and am perfectly happy with them.
Hella Supertones. ~$50-70. Disc-style lets you install in the grill, but will require a little more customization with installation. They are pretty loud, and sound great in person, however they do sound a little more..... electronic than other horns I've heard.
Wolo horns. ~$15-20/horn. Heard these on one horn and they sounded a lot like the Fiamm's I have. Install was similar, though the casing on the wolo's was a little more robust so it was a little tighter installing.
Stebel Nautilus compact dual tone air horn. ~$50. This is a dual tone air horn roughly the size of two cans of redbull. The overall sound isn't the deep tone you expect of air horns, but is very very loud. They're used a lot on motorcycles and I found a comment on one forum from a guy who was installing one on his bike before he rode home from work...... all his coworkers came outside to see why a train was going through the parking lot. Honestly if I were installing horns again I'd be putting one of these in. Another cool feature that some people have gotten is if installed directly to the stock wiring (vs. using the relay harness) it would go off as normal with the horn button, but when hitting lock twice for the confirmation beep the burst of power was so short that it didn't give the compressor enough time to fill and just gave a chirp instead of a full blast honk. However some people have installed it that way and still got the full blast, so it seems to be hit and miss.
