I don't know any instructors in those states, but honestly if you are willing to go from MT to OK or TX, why not just go to SE Florida and combine the trip with some good diving? That's what I did -- went with The Scuba Club in West Palm Beach, did my Solo class dives at the Blue Heron Bridge, then dove solo the rest of the week.
Although all of my other certs have been with PADI, I went with SDI because it actually says "Solo" on the card, not just "Self Reliant", which I've heard some dive ops don't regard the same (there's a thread on here discussing this topic). Used SDI online learning, and honestly most of the material it is not rocket science if you've been diving a good bit (as I see from one of your other posts) and generally take a mature approach to diving and risk assessment/avoidance (which I'm not sure of given your 300-500 feet statement -- nothing in a recreational Solo course will encourage that without proper tech training).