The answer differs for various countries:
Definitely at Sainsbury's, possibly at other supermarkets and specialty stores.
This site has some good answers about where to get quark or what to substitue quark with.
Here's a page about quark in Australia.
The above links can give you some ideas, but bare in mind that not all substites presented there are Dukan diet friendly!
You can also try and make your own quark.
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