I wonder how often public library keyboards are cleaned, or the ones in university computer labs. When I was in college pulling all nighters in the computer lab I regularly saw the lab being cleaned, but I can't remember ever seeing the keyboards getting dusted off, let alone wiped off with disinfectants.

Go to two or three libraries in different corners of town, get what you want from the keyboards, and you should have quite the collection of DNA.

With the ease of travel I'd suggest go to libraries in different states or countries. Libraries close to major airports so that many people are likely to travel between them. (ie don't pick a city in the middle of nowhere just large enough to have a private airport)

Fun activity: turn the keyboard upside down over a sheet of newsprint or butcher paper, and tap the front corner gently on a hard surface.

The word for this type of accumulation of mixed biological material, crumbs, dandruff, eyelashes, and such is "kroonch".