The rivets You are referring to are called Cutlery Rivets.
They are a male-female Compression rivet sett.
I do Not recommend You use them.
They require a Hole and a Countersink for the head and You must get the Depth Right on the First Try.
If Not--You will have handle slabs that are too thin or you will ground off the Thin Head by Accident.
Most Custom and Semi-Custom Knives today just use Epoxy and Pin Stock.
The Size of the Pin will be determined by the Hole s that are already drilled in the Tang.
You can open the Holes up but You will need a carbide Drill Bit to drill Out the hardened Tang.
The Corby-Style Bolts referred to above also require a Countersink but they are a lot more forgiving because you have a much thicker head that is meant to be ground Down-Flush with the Handle Slab.
I would suggest You just use pins.
This is Round Stock and You usually have your Choice of Aluminum=Brass-Nickel Silver-Copper or Stainless Steel.
Common sizes of this Pin Stock are .125", .156", .187", & 1/4th"
You cannot put them in a Hole of the Exact Same Size.
You have to purchase a Drill Bit that is a few thousands over the size of the pin or you can't get them into the Hole.
None of this is Rocket Science but it does help to have the Right Tools to proceed.
Let me know if You have any Specific Questions.
We do this about a hundred times a day so we probably can help with answers.