If you'd rather do glass, here are a couple of places to buy large glass jars in bulk. Half-gallons start at $1.33, full gallons start at $2.09.
Glass does break, but plastic is harder to clean, especially plastic with shapes. heat transference is another concern. Plastic insulates - but it keeps heat out as well as in. Glass transfers easily.
if you're heating them with a heat mat or tape, then plastic would likely be best, if you're going with the warm room/closet method, then you'd want glass.
A closet with a storm door on it, filled with shelves of jars of bettas is a very pretty thing.