Used boat hunting is a hobby of mine. For a tunnel boat boat like you're looking for Craigslist, 2coolfishing, boat trader and some of the bigger boat dealers that carry a large used inventory are probably your best bets. I'm sure Dargel and Shallow Sport down in the RGV get a decent number of trade-ins as well, may not hurt to let them know you're looking.
Craigslist is probably your best bet for a deal. 2cool and boat trader will probably have better maintained boats than the average CL boat, but you'll probably also pay a little more.
Set up search alerts in the major metro area craigslists to ping you when a model you're interested in pops up. Brownsville, Mcallen, and Corpus CL sites get the best traffic on the tunnel boats you're looking at, but they're pretty common for Victoria and Houston as well. Occasionally they'll pop up on San Antonio and Austin CLs sometimes for more money than a boat on the coast but often for less.
Shallow sports are going to cost you a premium over the others on your list but will probably also have a slightly better re-sale.
Seems to be quite a few 21' Majek RFLs on the market right now. That'd be a good Arroyo City boat, but maybe not as comfortable riding out of Mansfield or PI.
Whatever you do, take it somewhere and get it inspected unless you're very familiar and used to working on whatever type of motor it has.
Also, not sure about your budget / financing plans, but financing a used boat can be tricky depending on age. Paying for a cheaper boat with a blown motor (buying the hull) outright and financing a new motor can be much easier.