Most butchers will have a minimum, and certainly for the mixture you want to buy. They won't have mixed organ/ etc.
To start raw, it's generally advised to start with chicken/ bone only. This minimizes gastric upset until they get adapted to raw diet. Add in organ meats after.
For the trial, you may want to start with the commercial blends to see if it's tolerated. Then look for sources.
For cats, I'd invest in a good processor for raw only. Chicken wings and backs have the appropriate amount of bone, and rare often easily found. Buy chicken livers and blend together. For a cat, this is pretty inexpensive. I'd only head to a butcher for those small amounts if feeding dogs (which I do).