Two liters is half a gallon, right? This is an extremely small amount of water to support a fish. I highly suggest you upgrade to a container that is at least 2 gallons in size--that is about 7.5 liters. Most heaters are designed to be used in containers of at least 2 gallons and you won't be constantly under pressure to keep the water clean. In a .5 gallon container, you would have to change the water every single day to keep it clean--in a 2 gallon container, you'd only have to change it every 3-4 days and your betta would be able to have proper heating.
If money is a big problem for you, there are some very cheap options out there--the kritter keeper style tanks are only a few dollars, or another option to consider is a rubbermaid/sterilite clear plastic storage bin. A 2-4 gallon bin is only about $3 and they're totally safe for fish and safe to use with heaters. They're not very pretty, but it can help you get by until you can afford something nicer.
When you select a heater, make sure you get one that is 25 watts with an adjustable temperature dial like this one: http://www.fosterandsmithaquatics.co...8&pcatid=11368
since pre-set heaters and heater pads will overheat the water or not heat it enough depending on the temperature of your room--so chances are, a non-adjustable heater will just be a waste of money for you.
I would stop using Pimafix for now--pimafix is an antiseptic and a healing agent. It's useful in the same way that peroxide is useful when we put it on a cut, but if we had a cold, peroxide certainly wouldn't help us. The best thing for him is clean water and warm, stable temperatures. I would change the water every day in a tank that size, and move him to a place in the house that has the warmest, most stable temperatures (not near any drafty doors/windows/hot electronic devices/fans/vents, etc).