If you decide to cycle the tank with your betta in it, it will be exposed to high levels of ammonia and nitrite. To avoid these high levels you will have to have test kits that test for ammonia, nitrIte and nitrAte (you should have these anyway as they are an essential part of keeping healthy fish). You are going to have to test the water every day for these to keep the levels as low as possible (no more than .25 ppms ammonia and .5 ppms nitrIte)and will end up having to do small (15-25%) daily water changes to keep these levels as low as possible to prevent as much harm happening to the fish as possible. Depending on the size of the tank, it may take a few days for any ammount of ammonia to show up with your tests. You should never turn the filter off for any reason. This is where most of the benificial bacteria hang out (they hang out everywhere in your tank though, on the gravel, any decorations, and even the tank walls), and without a constant supply of oxygenated water they will die and you will have to cycle the tank all over again.
Most grocery stores sell pure ammonia in their cleaning section, but make sure that there is no additives and it is non scented. Some may contain quality controll agents but these don't seem to be harmful to the fish. The ammonia I used had these quality controll agents, but I did not notice any harm to the fish if there was any. I don't know where you live, but wal-mart sells ammonia. The name brand I used was Sea Mist. I have read on posts in the begginner questions section that a place to get ammonia in the UK is Boots(?). I live in the US so I'm not sure about that.