If I may suggest... you might want to consider a really nice large throw instead, something you can tuck into all the crevices and let hang loose around the sides. I say this because...
1. Slipcovers provide enough resistance to be a satisfying scratching surface for cats, so they quickly end up in shreds.
2. Good quality slipcovers are very expensive, and yet...
3. ...They still seem to look like... slipcovers,
y'know? Like an attempt to conceal, rather than merely protect. I feel like a well-chosen throw is more frankly casual and cozy-looking. And cheaper, and easier to wash!
IKEA has some very inexpensive cotton bedspreads of woven cotton in solid colors http://www.ikea.com/us/en/catalog/products/60104330
-- I stitched two of them together down the middle with a flat seam and they cover our sofa quite attractively! And best of all... the kitties don't enjoy scratching it, because it simply pulls up out of the crevices it's tucked into, providing no resistance.
We've also used those stretchy microfiber fleece blankets, but they tend to pill up and shed all over your clothes.