What lens are you using?
Basically, if you want cheap cheap, search "58 mm (circular polarize filter" on ebay, then do the same for just "polarize filter" and find one that doesn't rotate. Obviously, don't just do 58mm, buy what your lens takes or buy bigger with a step up ring (see below).
2. Also, what I like to do with filters is buy a 77mm. They cost more than a 58 or 67 or whatever, and I actually don't own 77mm glass but with a $2 step up ring on ebay they work with a smaller lens. You can't really put a 58mm filter on a 77mm lens because it will vignette, so by sticking to 77 I ensure that filters I buy will be able to be used on any lens I own or rent int the future, with a max $2-10 step up ring to fit the new lens. Of course, extreme telephoto lenses and matteboxes won't accept the 77, but MOST lenses I will probably use MOST of the time on my 7D will