I use almost out of date cream to make butter.
Whenever I see cream with tomorrows date on it marked right down in the shop, I scoop 'em up and make crudloads of butter to keep in the freezer. I culture the buttermilk and freeze that too.
What a great idea!
When I googled how to make sour cream alternative when I was desperate, I found a suggestion to add a couple of dessertspoons of vinegar and whip it slightly. It tasted great, I used the white wine one that was sitting around and I haven't bothered bothered buying sour cream since.