Just a thought - you can also make do in the meantime by using anything else that can add warmth, such as using warmed water, or finding a warmer spot in the house. Be very careful though not to create large heat fluctuations which can kill your fish - you'll need to use a thermometer to keep an eye on the water temperature. A larger container will hold its heat better too. Salt is cheap so it's not a problem to go to a larger container, just need to calculate the salt so you don't put too much in
If there's any way to grab a big container and fill it with warm dechlorinated water at around 27 or 28 degrees C, then you can slowly acclimitise your Betta to the warmer water in his bowl until it's at the same temperature, then introduce him to the new container. You can then add salt gradually to the right amount, and by monitoring the temperature and carefully adding (dechlorinated!) warm water, maintain the heat.
It would do until you can get to the shops and pick up a heater. At 20 degrees, your Betta is in survival mode, and the ich is going to compound the problems. Fortunately, warmer temperatures and salt both work to drive off and kill ich and the higher temp will help your Betta be less stressed and in better chance of recovery.