Just did about 75 rivets on a tunnel for snow-evac modification purposes. I just bought mine from Polaris dealer. The 3/16" structural rivets I needed from them are part number 7661637, they come in packs of 50 but I'm sure the dealer will sell them one at a time if you want, they are indeed steel, NOT aluminum, are a pretty gold color (not sure, but I believe they are gold because they are plated to prevent the corrosion that typically occurs when you mate dissimilar metals...like using steel rivets on aluminum tunnels), they list for exactly 27 cents each from Polaris, and are MUCH, MUCH stronger and MUCH, MUCH more difficult to pull than the typical junk you find in stock at a hardware store.
I'm not sure how many you actually need to buy for your single project, but I bought 100 for mine, and it obviously cost me $27 plus tax...without doing any negotiating. I have about 25-ish left over for the future. And based on the high quality of these rivets, I don't consider that to be horribly expensive. I probably could have saved $5 or $10 by wasting a bunch of time and effort shopping around, but would have ended up still not knowing for certain that I found the proper size, strength, or corrosion protection in whatever I found.
So my recommendation for someone doing just a single project? Just get them from the dealer. Of course you need to pay attention to the lengths and different sizes you'll need...may be different from what I purchased.
And as a side note, this pneumatic/hydraulic puller I bought on sale for this project at Harbor Freight for $35
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93458 works OUTSTANDING. Has a one year warranty to boot. I could not imagine trying to do this project without that thing. It jambed up once in a while and needed to be disassembled to get a stuck pin out, but it worked great for the price I paid. I tried pulling one of these rivets with a cheap hand rivet tool I've used in the past just for fun, and one of the jaws snapped in half on the first attempt...