I bought an alligator one from Dollar Tree a few weeks ago. Seen toliet seat covers at 99 Cent Only, Target pretty sure, Spirit Halloween, Halloween City. They really are all over. Try a couple of search on various store websites to find a theme you like best before heading out.
Ive bought them at party city previously, and spirit also has them. I discovered them at dollar tree a couple of weeks ago as well, but they seem a bit smaller than the others. Im a fan of the ones from party city, as they come with extra decals for the back of the toilet as well. The fiance wont let me do spiders in the bathroom, but I love the zombie one, and it comes with blood drips coming from the lid on the tank.