Where have you tried? I've got it from Sainsburys, Morrisons and Tescos in the past. For the life of me at first I couldn't find it but asked someone who was working there and they found it for me as it wasn't quite where I expected it would be. It's worth tracking down though as does make things lovely and creamy
A friend has been to Morrisons near Lowestoft this morning and got some there. She said it was slightly to the left of the hard cheeses in the soft/cream cheese section. That's the same place as in my local one so it could be the same at all of them.
I usually get it from Morrisons, in the chilled dairy section alongside the natural fromage frais, creme fraiche etc.
Half quark and half plain yogurt with a couple of eggs beaten in makes a great sauce for quorn moussaka and lasagne, by the way. I don't make them too often though because I can scoff the lot by myself.