If you have got good results with cottage cheese, I suppose that using it instead of quark or yogurt is not a decisive matter in the Budwig diet, the main thing being to mix oil with a sulphur-based protein.
But Quark is so easy to make that you should consider making it yourself.
I don't buy commercial Quark any longer. I now do it myself by straining organic yogurt through a coffee filter (unbleached). Results are better than Quark bought at the supermarket, and much less expensive.