It's your own preference on size of the tank, a 5 gal I wouldn't divide it more then just two ways. A 1 gal can be a suitable home as long as you do the proper 2x weekly water changes and it is heated 78-80F.
If you go 5gal and want to divide it, here is a step by step
way to do it for cheap. Normally you can buy dividers, but they tend to be for 10gals and up. There are different types of crafting mesh, some with bigger holes, some small, some white, etc.
For a 5 gal unfiltered you would want to do weekly 50% water changes with 1 100% monthly change. Always dosing the tank the full amount of water conditioner with the 50% changes. If filtered you shouldn't have to do a 100%, but do siphon every so often.
If you go with a filter make sure it is on it's lowest setting, or baffled
, or baffled with a sponge if the current is strong. You want as little movement as possible.
It's always best to use tap water over spring/bottled/distilled water. They are filtered to take out the minerals and nutrients your betta/fish need in their water. A water conditioner/dechlorinator
should take everything out that is harmful. Seachem Prime is the best one and comes very highly recommended.