You can buy 1.5" full port ball valves on line, or even from Amazon. I've seen them at smaller hardware stores for under $30, a bit pricey. I know HD's don't stock larger valves.
If you don't have a bi-metal bit that large you can simply trace the hole size onto the drum from a 1.5" nipple you'll be using. Drill a starter hole and you can use a zigsaw to cut out the hole.
I like the single big valve set up but the parts alone can cost you close to $60 with a 2 foot extension, for instance......something to consider.
....UDS....Mini-WSM....Midi-WSM....Mod-ECB....22.5-OTG....34"wood fired pizza oven....