Black file folder, or sheet of plastic, (Hobby Lobby, Michaels, that kind of place) will do fine. I use a piece of black plexiglass which is good for someone who needs it more often. I tried cloth, duvatine, leather and textured plastic, glossy black works the best.

Since I use Adobe Elements/Photoshop I can adjust the Levels, click on black dropper and a grayish black spot on the background. If the stamp has white, white dropper.

Photoshop Elements comes free with many cameras. Even version 2 (the one that Canon used to ship with their cameras) will do levels. Basically you are setting the white point and the black point, after the fact.

Don't recall but these probably started at 1200 dpi TIF. 600 dpi jpeg is more than good enough, depends on how much detail you want.