Whats the best way to search for a nation wide TC purchase?

In general focus on the PNW and mountain west states. That's where the vast majority of TCs are and also the right side of the country for you.
Personally I'd go private party. Can't trust a dealer anymore than Joe blow so why pay the premium. It's buyer beware anyway.
Craigslist seattle alone probably has 100+ TCs any given day. And if you know or can find a rvnet buddy to check it out first when you find the "one". That's a bonus.