Yeah, I first saw those Zentangle mags in Tesco and now they have loads of the Zen colouring mags too. They usually have about 4 or 5 different ones in but right now they have more. They also have a couple of books in the book section which are handy sized but there are plenty on Amazon too.
I set up a thread for the Zentangle here:
http://www.nomorepanic.co.uk/showthread.php?t=168827
Lior posted a good free guide on how to draw them in there to save buying one of the "how to" mags. There are some very interesting ways you can meld styles, I posted a few examples, and there is no reason why you couldn't colour these in either.