There are a wide variety of loupes available. BorrowLenses.com has the Hoodman and Zacuto available for rent.
Currently, I use the Hoodloupe by Hoodman. It's a simple design with a diopter (allows the finder to be adjusted for a user's eyesight) and a couple of mounting options. The one I use is the crane. It allows for precise adjustment of the finder's position and also let's you quickly remove and replace the unit. It requires no adhesives or anything like that. Brilliant design.