The AK doesn't use a roll pin, they use a solid pin that's riveted in place.
Many people just use a section of common nail. It's soft enough to easily rivet and strong enough to do the job.
You lightly countersink each side of the hole in the bolt carrier to give the rivets room to flow down into the countersinks, then rivet the nail on each end by tapping with a hammer to rivet the ends.
After riveting, grind the pin flush with the carrier tube. The countersinks insure that once you grind off the excess mushroomed ends, there'll be enough rivet left to hold the pin in place.
You want the hole through the gas piston slightly larger then the pin so the piston will have just a little wobble. This allows the piston to align itself with the gas block.
When installing, screw the piston in until snug, then back it out about 1/2 turn to allow the slight wobble.
If you just want to use a roll pin, go to any hardware store and buy about a 3/16" roll pin.
Again, make the hole in the piston slightly larger to the piston wobbles a little.