I have a few different worm cultures growing. They would be fine for feeding betta fry. There are micro worms, banana worms, and walter worms. I bought my cultures off of ebay, and as long as you follow the instructions the "worm dude" sends you, you will be fine.
The easiest way to go about feeding fry, is to find Hikari first bites. I learned my lesson last time with brine shrimp eggs. I am now having a serious snail issue.....