A few breeders use the beanie bear boxes and they hold 2 litres of water
It doesn't sound a lot but these guys are so small when you jar them that they have plenty of room to swim around in. The last of my juvies are still in boxes and they are 4 months old now and still have room
Take a look in the pinned topic for breeding, Wuv has some excellent tips for breeding and raising bettas
I am using Pickle jars, and canning jars I had left over from Oct. when I made applesauce..You can get canning jars really cheap at Walmart in housewares, and they come with lids - about 10 or 15 to a pack. About as big as those plastic Hexagon tanks they sell at your local petstore, sept they are glass.