Personally, I would just get a new 2-4 gallon rubbermaid/sterilite bin from walmart or target. This bins are only $2-$4 each and make excellent hospital tanks because of their long, shallow shape--ideal for sick bettas who may not have the energy to swim far to reach the surface to breathe. These bins are big enough to be heated and aerated, and are light and durable for ease of water changes. Many come in clear plastic.
I agree with Adastra. When looking for a good hospital tank you want something that is shallow and wide and preferably 5 gallons. The reason you want 5 gallons is that it makes it much easier to dose medications that way. You also want a tank big enough that you can easily heat it.
If your just looking for something to use as a quarantine tank then a jar works great. I use gallon pickle jars for my QT fish. Just rinse it out with some water and vinegar and rinse it again and you should be good to go.
My fish has currently ripped up his fins. He seems to be healing up pretty well so far. He's still really active, but I thought that maybe putting him in something smaller would make it easier to change his water and also to minimize the amount of medication I'm using.
The jar that I have is a 1 gallon jar (I checked today). It was washed with dish soap a few months ago and has been sitting on a shelf since then. I was planning on setting it up with a small Hydor mini heater and making sure that I monitor the temp really well.
Adjustable heaters are only a few dollars more but are infinitely better than the heater pads. I still maintain my earlier suggestion about getting a bin as opposed to the jar. You could throw the plastic bin however you wanted and not have to worry about it breaking unlike the jar. In this way it would be more practical for frequent changes.
I like to use containers that are clear so that I can see the fish during treatment and have found glass to be easier for me to view the fish through. Observation of the fish and waste produced in all angle/direction are really important during treatment to determine effectiveness/healing...etc....