Here is a keyboard tray:
For someone who is short, and works in a typical cubical in the United States (where the desk height cannot be adjusted), raising the chair will stop your feet from being properly flat on the floor, which causes back problems. Patricia Shanahan's link to the Mayo Clinic guide is a good one to understand proper configuration, but higher chair with a foot-stool is not ideal.
In my office, I am the only one who is short enough to need a keyboard tray. I requested one from my company, and they installed it for me (it is one of the adjustable ones you bolt under the bottom of the desktop). I've been comfortable at work ever since. If you have a keyboard-centric job, it is reasonable to ask for this. I have had some pushback at other companies when asking for ergonomic equipment - I recommend doing a little research if they are reluctant, so you can come back and point out the potential dangers to your body long-term from a bad setup. It might also help, if they try the "now everyone will want this equipment" argument, to point out that no one else actually needs it, so they don't need to worry about everyone asking. I had to do this when asking for an ergonomic mouse after everyone in the office had seen me wearing a wrist brace, and pointed out that the potential loss of the use of my hand was much worse than the price of an expensive mouse.
Footrests are also an option, but having tried this with other employers, they do not work as well as a properly configured workstation with an adjustable keyboard tray (and monitors lowered to the proper height). It's easy to kick them, move them around, stub your toe on them, etc. If your company absolutely refuses to help you though (which can happen) you might be able to bring in a box for your feet. Try to get one that is very wide so you can move your feet around a bit, since most foot rests are pretty narrow.