Hi stewartsthebest
I'm a big fan of feeding live foods, but many people do well with the frozen. Most good lfs have live food which is sold by the ounce or "portion" which is usually less than an ounce. I pay $2 for an ounce at one lfs I go to and $1.29 for a portion at another. These are for blackworms or tubifex worms. Either one are good for a betta and mine love them.
The important thing to know is that they have a limited life span, so get just a little bit at a time. Because they are in a little water and they excrete in that water, they must be washed daily to keep them safe as food. This isn't hard to do.
I use a liquid measuring cup to wash them because it has a little spout which makes pouring off the water easier. Just take the worms from the little cup or baggie you bought them in and pour them into the cup. Add cold tap water and stir them, making sure they separate from each other. The water will become cloudy. Spill off the water and repeat until the water runs clear. Then they are safe for your fish.
To store them, pour off the excess water so that there is just enough to cover them and put them in a covered container in the refrigerator. Clean them like this once a day and they will be safe food until they die. Then throw them away and buy new ones.
The little bit of trouble it takes to prepare them is well worth it, in my opinion. Your fish will love you for it.