There are other options beyond katzeye. Focusingscreen.com has a whole slew of possibilities.
Or if you're feeling really adventurous, you can cut down your own screen from most Canon EOS screens as well as Pentax SLR screens.
If you don't wear glasses, you should also get an eyepiece magnifier. If you do wear glasses (like me), you're best just practicing (or getting a flip up 2x+ maginifier and knowing you'll lose full field of view).
Also, if you aren't in a hurry, live-view provides the absolute best accuracy. Just push the Lv button and use the INFO button to change the zoom level (and the cursors to move around). This works excellent for tripod work and still-life/studio portraits, but is too slow for action, street shots, or chasing children.