You can get one from CDR King for Php100.
Here's what it looks like to give you an idea.It's plug and play for almost all versions of Window so no installers are needed.
It works with ExpressScribe and a host of other transcription player software as well.
Simply plug it to your USB slot, open ExpressScribe, go to the "Options" tab, select "Controller" click "Controller Set Up Wizard" you'll see that it will be detected as "generic game controller." click the necessary buttons until you reach the Command/Controller Map Test. Set buttons 1, 2, 3 and 4 (the ones bundled in the left) to "Play"
And presto, you have a single-step USB foot pedal.
You can also set it to have ffd and rewind functions but the buttons are too small, but if you have a bit of electronics knowhow and a soldering gun, you can hotwire the buttons to three external switches that will serve as your rew, ffd, and play buttons for more comfort.
Hope this helps.