If it was plastic, measure the opening to get the size measurement you need. Finding OEM caps is going to be an E-Bay thing. Next best is to either look at all the cheap pedals for a cap size that fits (price of the pedals will be cheaper than the E-Bay price for the original caps and pedals don't use that many different sizes or shapes of plugs), or look at the various plastic plugs that are available and pick the right size. One of the standard sizes should fit. Some companies even send out free samples:
http://www.stockcap.com/styles/plugs/index.asp
Check your local auto part stores and there are rubber and plastic plugs in the 'problem solvers' section. Some art, hobby, drug stores, and X-Boxes have plugs in various sections around the store.