Not sure what Opps rivets are, but you can order "Oops" rivets in Vans webstore, with quantities as low as 0.01 lbs, which isn't too many. They will likely send them in an envelope too, so shipping isn't much. I ordered 0.01 of several sizes early on so I'd have them throughout the build.
Van's stocks both the AD3 and AD4 sizes in several lengths with prices that are much lower than Pegasus. You used to have to ask for the AD3 rivets, but they're listed on the store now.
Per Gil above they are NAS1097 AD4-xx rivets and are in stock at Aircraft Spruce. I have a handful of different lengths and they came in handy when I was learning how to drill out rivets. Sometimes the hole got a bit too big and had to be enlarged to take a 1/8" shank but still keep the 3/32 head for the existing dimple.
NAS1241AD rivets are 1/64 oversize (1242 for universal head) NAS1097 are next size up with reduced head(#4 rivet with a #3 head) Why blow open a hole the full next size when someimes you can go half way?
Genuine has both.