What pan can be subsituted for a Paella pan?
You can use about any pan that you could cook rice in. I've made Paella in a stock pot, hotel pan, and a Dutch oven. Honestly, the paella in the Dutch oven turned out better than it usually does in a paella pan. The heavy bottom of the dutch oven allowed for a much better layer of toasted rice at the bottom.  For very large quantities a hotel pan is most convenient, but you may have better luck cooking the rice separately and tossing it together with the other ingredients at the end.