Quark is similar to sour cream. You can make your own using purchased buttermilk.
Recipe uses nettles for a natural thickener or use rennet tablets.
From the message board of CountryLife.net here are two methods for making your own sour cream at home.
Recipe for making homemade blue cheese. For detailed, illustrated instructions please view David B. Fankhauser Blue Cheese Making Page:
Here are two recipes for a substitute for mascarpone cheese. The whole milk produces a luscious light cheese. The tangy flavor is ideal with a sprinkling of chopped chives or seasoned pepper on appetizer crackers, or with toast and berry jam for breakfast.
Alternately, the whipping cream version makes a velvety, cool tasting cheese comparable to the Italian mascarpone. Serve it ...