Try this website. I think you will be able to find something to fill your needs.
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.