Try this website. I think you will be able to find something to fill your needs.
www.emhart.com/products/rivets.asp
This manufactures website offers one of the most comprehensive assortments of pop rivets and information I've found. Another is Alcoa/Huck (marson).
You can usually order these from McMaster.com or MSC.com.
I especially like McMaster as they have very competitive pricing, thousands of items in stock, and if I order by 5:00PM I usually have the item the next day without any extra shipping. Their online catalog has a wealth of information in it also.
This absolutely not an advertisement for either, but just a couple of good hardware houses.
Mark