I have a few of these. Some of the names I've heard them called are Removable rivets, Plastic Rivets, Push locks, plastic pop rivets etc etc depending on the brand. Check the hardware store first then automotive shops, they may have some or be able to order then for you.
http://pcmods.com/details.asp?ProdID=391
The are currently on backorder. Other mod retailers may have them in stock.