The Bob Wagner book has some great information on indexes. Personally, Bob shared a bit of advice that I think is great along with what David has stated above. Bob told me to visualize the position/location of the card before going to the pocket to remove it. I found this to be quite helpful.
Long ago, Charlie Miller featured my index in his Magicana column. It was useful for those that wanted to produce a card from the side pocket of your jacket. Cheap to make and "easy" to use (with proper practice).
Ron Wilson and Charlie Miller also released a small pamphlet on their own card index. Perhaps you can find that on the secondary market.
Good luck in your search,
Paul (Happy New Year) Green
:D :D :D