Tubifex worms are another larval stage of something. They are typical of polluted waters and are sold in freeze dried cubes which makes them less suitable for bettas in jars.
There are other freeze dried foods such as daphnia and brine shrimp but I haven't had much luck with them.
Go out after a rain and pick up some earthworms. If you aren't too dqueamish cut them into little bits and feed them to your bettas-They will love you for it :)
A betta's natural food. In most areas all you need to do is set out a jar or bucket full of water and you will eventually get some mosquito larvae. Net or pour into your betta's jar and he/she will be exstatic.
One way to get these is to go out to a creek and seine out some newly hatched fry of one of the common species chubs etc. Of course you could always just buy a female guppy or three and feed the babies to your bettas. Some might consider this cruel but in nature most baby fish get eaten anyway.
This is a weird one but the fish absolutely love them. In the fall when the grasshoppers are laying their eggs catch a few cut them open and feed them to your bettas. If you are fishing in the spring and you are filleting you catch and you find a bunch of eggs don't throw them away, feed them to your fish.