The only physical ingredient involved in the making of butter is cream. Indeed, the process of butter-making doesn't involve adding
things to cream as much as it does taking them out
--to wit, buttermilk. We got our cream at Costco
. and I think the price was something on the order of $6 for a half-gallon. Figure on getting about half the cream back as butter.
Also: try to buy a cream that is as unadulterated as possible. Some commercial creams contain carrageenan
(a natural thickener derived from a type of seaweed native to the Irish coast) or other thickening agents, and while I don't think it'd be impossible to make butter in the presence of such, the end product wouldn't be as good. You don't need to go down to Farmer X's stand and buy the cream he got from yesterday's milking (although butter made from such cream would probably be excellent); you just need to get something that has in it more natural product than plastic.