I bought this pop-up diffuser from eBay for around $11 US including shipping. Do a search on ebay for "Diffuser for on-camera flash/speedlight" (or click here to do the ebay search). The seller seems to have quite a few of them for sale.
I had to modify mine slightly in order to make it stay on the lens. I glued a wide rubber band to the inside of the opening that attaches to the lens, so it would stay on the front of the lens where the lens hood normally goes.
The next image shows my modification.