1) Go to your food store and in the deli place ask if you can buy a sleve of 16oz deli cups with lid's the only bad thing about this is the betta in a cup that everyone hate's when they walk in pet store's if i do store them in a cup i float it in my 29 gallon so that it keeps a good temp and i will not leave a betta in it for more that 7 days it isnt permanent housing for them plus they dont continue to grow as fast as if they were in a gallon or more.
2) This one a lot of breeders use you order the bennie baby case or box they are clear like a plexi glass tank and there a little over a 1/2 gallon this is how i store bettas to be sold and each day i will siphon from the bottom get all the trash and poop and then keep on draining the water and do a 90% water change each day this still lets juvie bettas grow at a good speed they have room and good water everyday and the cost of this set up isn't as much as #3
3) So this set up is used by bigger betta breeders or someone that can afford it and cares about there fish a lot i have a small one set up with 20 bettas right now. It is a betta drip system you get some shelving and pvc pipe some 1/2 inch tubing and a submersible pump and some plastic gallon containers with lid's the containers that are clear is the best one but most of the time they have a little frost to it you can still see the betta but just not clear as an aquarium anyway you set it all up and you use rubber air line tubing with 2 lines out the bottom and 2 threw the lid so the water will slowly drip out and down to a 40 gallon container where it gets filtered and then pumped back up and dripped in threw the lid so theres always fresh water and that is key to a juvie betta growing at a good speed if you really want the drip system email me and i will email you the paper that shows you how to set it up