I just ordered a Tifton Hardy from Plant Delights in NC. My baby arrived healthy and happy. Mine was a little smaller than you are looking for. You may be able to give them a call to see what you can get there from them. I do have some of the other cold hardy palms, Jelly's, Fan and purchased in the sizes you mention in big box stores. Palmetto's are one of the cold hardy palms that will grow well. Just remember to mulch well and take recommended precautions against freezing in the winter. Good luck :-)

Never seen solar or battery power. Mine is large enough that I can put two standard size seed flats on it. I haven't noticed the electric bill difference. Of course, my wife pays all of the bills. Make sure you buy a thermostat for it though. Otherwise you will be pumping out 110F or so which usually cooks whatever you are growing. Cheap online, try amazon or google shopping.

I bought a small sabal minor (is that the same thing as a sabal palmetto?) that has been in the ground outdoors here for three winters now. It is in a sheltered area near the house wall on the south side. I don't protect it much, just a little bark mulch on the bed like all my other plants, and a spray of Wiltpruf in the fall and maybe once in the winter if the weather cooperates. I get some pretty severe leaf burn when the weather is nasty, but I cut them off and it grows new ones. Of course, it hasn't increased in size much. I think more winter protection and more sun would help (it gets shaded a lot in the summer once the bananas get all).

I put mine in moist sand and store them in the bottom dwawer of the fridge until spring (a dip in some bleach water first won't hurt), I figure this simulates the natural conditions here on the gulf coast. I saved about 50 seeds this year but the last time I germinated them was about 5 years ago, the ones I germinated then are just now getting their full leaves. I use just ordinary potting soil and black nursery pots, don't worry about using that fast draining palm mix, Cabbage palms are not picky about moisture. Once they get going make sure to move the pots often or set them up on some blocks or something, the roots will go through those drain holes and be in the ground in no time.