Building CAM870 shotgun shells?

Hey everyone, I'm considering a CAM870 but the prices on the shells are something else.

I noticed on Evike that shells are $18/4, but to buy the shell casings and end caps would equal about $13.50/4, since the caps are on sale. But since I'm looking at a used gen1 model, the shells would hold the propellant, and I'm pretty sure the caps do NOT come with the pressure chamber needed for them to work.

My question is: has anyone found these available for sale? I've looked on Evike, RedWolf, and even APS's website but haven't seen them available as replacement parts. Links and personal experience would be great to see!