Hanson Rivet's tubular rivet page has some info on length and clinch allowance. It's pretty straightforward. Measure the thickness of the two pieces of material, add them with the clinch allowance gets you length. If they don't have that length on stock, you might need to order 100,000 of them and wait for them to manufacture them.
I don't know what their minimum quantity is, but I bought 100 of the ones I ordered, just to have plenty.
