Hi rollntider
I'm sorry to learn about all the trouble you seem to be having with your bettas. They are such lovely fish to keep with corys, too.
IMHO, I think that most bettas are more of less sickly when they are purchased from a lfs, unless those bettas have been freely swimming in their tanks for a period of time. Of course, with this kind of thought, all of the most attractive bettas in a shipment will have been sold before you get a healthy one.
What I think happens is that they become infected with the Columnaris bacteria during shipping. Even the best lfs often gets their delivery of bettas packaged in tiny little individual packets. These have barely enough water to cover the fish and the rest is air. This makes for a light shipment which is important to them to keep the costs down.
So, by the time the fish arrives at the lfs, they have been under extreme stress and their immune systems have been compromised, leaving them susceptible to this bacteria. From this, they may show symptoms of finrot, "fungus," or just a general shabby look. On the other hand they could be infected and still appear fine. Then the lfs unpacks them and puts them into different water. I doubt if many of them are slowly acclimated to the local water like the fish that come in larger bags do. This is another shock to their systems. Then, when a customer buys them they are moved into still another environment, and by that time they are sick. Symptomatic or not, they often seem to die within the first few days of purchase.
I see this all too often on the forum.
Many bettas go to their new homes and live in tanks of their own that can easily be given good and frequent water changes which will sometimes be enough to get the fish through their difficulties with no other treatment. IMHO, even if a betta is destined to live in a community tank it's a good idea to isolate them for a period of observation and/or medication both for their own good and for the good of the other occupants of your tank.
BettaMax is a good product to use at this time to heal them and rebuild their immune systems. (Unfortunately, it seems to be disappearing as Aquatronics products are being sold out of many lfs.) MelaFix might work well at this time, and I'm sure there are other products as well. A temperature of no more than 75 degrees F. and good oxygenation will also aid healing. A good nutritious diet is also a must.
Personally, I think a lot of getting a good, strong and healthy betta at an lfs is just dependent on luck!