Auto body/paint supply stores have some of them Auveco (?) has several sizes in their catalog. Others you might need to find a repop parts store for that particular car and buy them.
Fastenal has some like that. $1.00 each after taxes. Sold by the box only. 50 per box. Nuts are extra.
There's a guy I've dealt with that I'd like to reccomend. Rick's Automotive Hardware in Vermont. He's not a big operation,but his deals are pretty good. Check it out at: http://automotive-hardware.com/
http://www.clipsandfasteners.com/Moulding_Fasteners_10_24_X_3_4_Bolt_5_16_Width_p/a12962.htm This is a good place.
AuVeCo makes them all but does not sell directly. Pretty much all the vendors noted in this thread are selling AuVeCo parts. I get them at Carlisle if I remember, or at my local auto paint store otherwise.
Here is the entire AU veco Catalog on line, If you can't find it here, you don't need it! It will take time to load. PDF FILE 649 pages. Down load, and burn to CD. http://www.motorcityfasteners.com/AuvecoCatalog.pdf Use the icons in the left window to access the menu. In the margin to the left you'll see an icon that look like two pages on top each other, right click on it. You can pull the left window edge to the center, and enlarge to see the details of every item. Scroll thru to the item,that your looking for. If anybody can, explain any better, please do. http://www.motorcityfasteners.com/AuvecoCatalog.pdf
My local automotive paint store has a display with about a thousand different fastners/clips/, including some that look like that.
Hillco fasteners.. City of stanton i believe. (714) 657-7442. Walk in and get what you need. They have tons of stuff
If you just need a couple, try your local body shop, they should have a few. Otherwise get them at a body shop supply.
They are not clips but called molding bolts. Any rate, you can buy them here... http://www.restorationspecialties.com/ Catalog shows actual size so you can match them up.