The lip is just there to center the washer to the hole thats in the diffuser. Its not functionally important enough to worry about using just a regular fender washer (that costs $0.30 at hardware store...Ace Hardware will have some chrome ones that will cost a couple bucks if your worried about rust).I just removed my undertray and so have them at hand. They are 8mm x 25mm. I am not sure what the thread pitch is but can find out Monday if you let me know you still need it.
The washers are special. They are 38mm in diameter but have a step to them that is about 2mm high and 12.5mm in diameter.