what are some good, deep, yet reasonably priced air horns that will not require me to forfeit my stock horn... i just want an airhorn for shows/personal pleasure

If you want true air horn(s) you'll have to have an air compressor and an air tank to operate the horn. You can go with the electric style "air" horn but they are not as loud. Your local semi truck dealer would be a place to go or online shopping.

Fiamm makes some really great air horn systems.
In the U.S. they are sold as WOLO.
Good stuff but they do have a rather high tone on some units.
Your other option is to look at older 5 ton trucks and mount one of them.
I had one out of a 5 ton in a Mini and it did the trick!
Only paid a fiver for it as well!