Try finding a long tank. My petsmart has a 15 long for 30.00.... 25 long for about 45 or so. no hood however. Second hand stores are your friends!
A 20 LONG can be divided 5 giving each betta 4 gallons.
Heating each individual jar will be costly, hard, and not worth the trouble. I've heard of heat pads being used, but they use a LOT of electricity... I heat a room for my bettas, which then the room itself is at about 80.
When I have "too many" bettas, I divide my largest tanks into as many as possible, providing as much cover as possible for each betta, to avoid them getting stressed from their neighbors... OR I use coroplast. Sturdy, though you could use a little silicone to secure the top. Then this way they do not see each other, however there won't be air flow from a filter throughout each section.
I think jars should be kept for breeders, and sick fish. Not kept permanently... Unless for some reason a betta has a chronic issue, like Chronic SBD, but then a shallow bowl would be a better option.