This happened on our BMW, the replacement part was relatively cheap, but they aren't painted, so a body shop had to paint the plug to match. Not sure if Toyota works the same. We were able to convince the BMW dealer that it should be replaced under warranty, since it literally fell off the car without anyone touching it...but the car was brand new and I think they just did it to keep us happy.
Yes, it looks like it only comes in black for the Prius as well: 2004 Toyota Prius Black - Parts.com For most parts, it's cheaper to get them from online discount vendors. But here, where the cover retails for only $5, maybe it'll be cheaper from your dealer if the dealer doesn't charge you shipping costs.
