It is illegal to legally hunt with airguns down here. So, your only recourse to do this is to find someone with private property and a iguana population that needs eradication. It is legal to shoot non-native species on your own property (provided it is legal to shoot airguns on your property by local statutes in that area).
Iguanas tend to hang in populated areas along the coast, as there is minimal natural food sources compareed to native central america. From what I see and read, on the east coast, they are around West Palm Beach and south tthru the Keys. On the west coast, Marco Island, Naples and Bonita have issues with them, and the island of Boca Grande is over-run.That's as far north I know of. Liscensed trappers either live trap or snare them and shove them in the freezer to put them down. Of course, the tree huggers are upset about that.