RE: Christmas Pudding
You can get suet anywhere beef is butchered or you can start now trimming some of the white opaque fat off rib and strip steaks and saving them up in the freezer. Then just grind or chop up your trimmings to use in your recipe.
A note about the mixed peel: The majority of U.S. grocery stores stock candied fruits and peels only during the Christmas season. If you wait until October, it should be easy to find and only a fraction of the price of ordering it online.
Have fun with your Dickensonian Christmas treat.