Yeah, you can buy thc-a hemp legally in most places, as it's federally legal, and few states restrict it specifically.
But.. just a couple notes to keep in mind:
This is indeed a grey area, even if it seems explicitly legal. Thc-a, through time, heat, oxygen, etc, WILL degrade into federal illegal THC. So if you buy a jar of thca hemp, the thca will turn into THC over time, and potentially rise above the legal limit of THC percentage.
This could mean, in the worse case scenario, that if they confiscate your legally purchased hemp, and test it, then they could charge you for illegal cannabis possession because it has more than the allowed thc, and therefore is not legal hemp.
Also, I've asked at hemp stores, and was told no.. But indeed, some growers spray thca liquid crystal and even non-cannabis derived terpenes on their bud. Pretty shady imo..
This market, although a legal victory, is actually a detriment to the industry and legalization as a whole, IMHO.