I have two different slipcovers. One is from surefit and the other is from Pottery Barn, that one fit my couch even though I do not have a pottery barn couch.
Anyways, I would try Surefit, they have a lot of options that would likely fit your couch. Constant tucking and such can get annoying, but a slipcover that is fitted, and has a separate seat cover helps a lot. And, if you can sew, you can also sew up any loose edges. Surefit also has tips on getting the best fit for your couch... and I would also check their return policy in case it doesn't fit.
This is the Surefit one I have:
http://www.surefit.net/shop/categories/sofa-loveseat-and-...
(you can see it here on my couches)
http://littlelovables.blogspot.com/2010/09/homefront-livi...
And here are my white Pottery Barn ones on the same couches, along with instructions on making your own custom slipcovers:
http://littlelovables.blogspot.com/2010/03/white-slipcove...
To save money, I got them from amazon and ebay.
The issue is your cat though, he/she will just shred up the slipcover too. We trim our cats nails which is quite effective and we also have scratching posts, and spray her with a water bottle when she is naughty. When we aren't home, I put her away in the bathroom or my room so she can't have her heyday tearing up the furniture. You can also get softpaws put on your cat.