I have an '07 Prius as a dog car as well.
I have a set of GT covers that includes the front seat bottoms and backs, rear bench bottom and backs, center console cover and small covers for the side bolsters on the rear bench seat. They are a pretty good fit and are very custom. They even have pockets on the side of the front buckets to accommodate the side airbags. They take some work to install and require removing the rear seat bottom, but it's not as scary as it sounds--it has a few snaps holding it in. The instructions are pretty good. They've been on since the car was new and are holding up nicely to our high energy shepherd mix and red heeler.
Unfortunately, they've gone up in price a lot since I got them. I got the whole set from their Ebay store in Spring of 2007 for $35. Yikes, now it looks like they're $324 for cloth.
Here's their site:
www.gtcovers.com
You could also find some generic covers at an auto parts or department store but they likely won't have the custom fit. If I needed covers now though, that's what I would do--I couldn't afford the new prices on the GT's. Just make sure they have openings on the outer sides of the bucket seats so they don't interfere with the side air bags.