I think The Cover Project had a forum post about this as well, but I use a semi-glossy paper from xpedex that's 8.5 x 14 (legal size), and then always make sure to print from Photoshop with crop corners.

If you're going to do a lot of covers, I highly recommend a cork-backed metal ruler to line up the crop marks, and a comfortable x-acto knife with several blades. Also make sure you've got something underneath to protect the surface you're cutting on, I use an old cereal box that's been broken down.