If the betta is the only resident in a 5 gallon tank, it should be ok. Make sure you test the water for ammonia and nitrites for the first month or so he's in there, you may need to do more water changes than normal. If you want the healthiest fish possible you shouldn't expose it to ammonia or nitrite at all imo, since it can cause stress and permanent damage which leads to disease later on, but just one betta in 5 gallons is probably sufficient dilution that he'll be ok if that's what you really want to do. As far as ammonia source, that depends on your location, people in the UK seem to be having trouble finding it. In the US it is commonly available at Walmart, drugstores like Walgreens, grocery stores and hardware stores. Make sure it is "clear" or "pure" ammonia that doesn't foam when you shake the bottle, although a few bubbles are ok.
