I got mine from a local home brew shop. I used champagne yeast. It was quite a bit more expensive than bakers yeast, but not too terrible. The sugar was a comparable cost, per batch of DIY Co2. Granted, I used 2 cups of sugar, and no more than a tablespoon (can't remember the actual amount off the top of my head) of yeast per batch...
Just be sure to get actual yeast intended for home brewing, and not "brewer's yeast". Brewer's yeast, the common food additive, is actually deactivated yeast, and you need active yeast to make DIY Co2 work.