Practically any electronic component store will have something that will work.
Most likely the knobs are a press-fit, and the shaft of the pot will either look like a letter-D when you look at it end on, or it will have splines (grooves). You just need a knob that is suited to the pot shaft and is similar in size to the original.
Just take your pedal to a store and try knobs from the display trays. You need an electronics store that sells components, as opposed to electronic consumer goods.
Hope that helps.