You can contact most any of the current tinsmiths advertising in the magazines or through web site links. There is also a tinsmiths organization you might want to check out. I don't know if you will be able to see much on their web site until you join, but here's the link
Tin Tinkers
There are also two types of tin plate out there - modern electroplated sheets, and hand-dipped sheets. Most of the tin stuff you see nowdays was made with that modern electroplated stuff. It's good stuff and pretty smooth/consistant, but is a kind of thin layer on the iron. The hand-dipped sheets have a much heavier coating/plating, and it isn't as smooth or even a coating. But the hand-dipped is much much closer to what was originally used back before the late 1800's.
One note about those "machines" most tinsmiths use to make their items. Those machines started to come into production and use in the mid 1800's. So the types of seams they create are kind of different from the hand-formed seams in earlier items.
As I recall, Bob at Backwoods Tin is the current prez of the Tinsmith group. If you search for his web site, you should be able to then ask him for some sources - especially if there are any near you. He also would have both types of tinplate on hand. But if you locate a tinsmith near you, they could be talked out of a sheet or three. Or if you are going to an event and you know a tinsmith is going to be there, contact them and see if they will bring a couple sheets along.
That Northwest Journal web site is great! They have that Making your own Copper Pot article, step by step and with pictures. A good source for some copper sheet to make it is the local antique store. Go and find one of those old large oval "wash boilers" to use for material. You should be able to get a good deal on one that is a little beat up. Or get some scrap sections of roofing copper from a local contractor. They always have short sections or odd pieces from a roofing job. Altho, scrap copper prices have been pretty high lately.
Hope this helps. Good luck on your "tinsmithing" project.
Mikey - yee ol' grumpy German blacksmith out in the Hinterlands