please realize that we're not trying to pick on you- it's just our opinion that such tanks are too small and we say the same to everyone asking about them. the general rule with all fish is 1 gallon per inch of full grown fish- and that goes for betta too. since a male betta usually grows to a little more than two inches in length he generally needs at least 2 gallons to get exercise and such. you can see here if you click the link the fish of a regular poster- Zeus- making use of the space he has. you can see that he's an active swimmer and not just lying on the bottom of the tank- as they often will if they don't have enough room. http://www.youtube.com/watch?v=0q1uRqQ_efY
The smallest heater (I've researched this extensively in an effort to heat my 1g bowl) on the market is the Hydor Mini Heater. You'll want 5 watts per gallon, so a 10w like the Marineland Stealth would heat 2g, a 25w heater would heat 5g, etc. A heater is a must!
PS. If there are any in Montreal, I always find pretty good prices on equipment at Mr Pets... the fish are pretty mangy but the supplies are definitely decent!