I thought you wanted to save bettas, not breed them?
Look in local classifieds and Ebay for used/secondhand tanks. Length is more important than height/depth. Get a 4 foot long tank and divide it into 5 gallon sections. You will take up less space and power points/plugs if you get a long tank and divide it than if you get loads of little containers.
If a long tank is not a viable option for you due to space, then go to a hardware store or something similar, and pick up a few clear plastic storage boxes of the sort you can slide under the bed or stack. You can find them easily enough and they are usually pretty cheap. They need to be at least 12 INCHES all around , and at least 8-10 inches deep/high.
You can set those up as individual tanks, but you will need extension plugs in order to be able to plug in the filters and heaters, which are essential ( having a labyrinth organ does not excuse these fish from requiring filtration )
Sponge filters are perfect for bettas, as they produce no current. They require air pumps to power them . Breeders use them in fry tanks when breeding so they are perfectly safe for weak swimmers like bettas.
You will need heaters for all the tubs to keep the temperature constant.