Definitely get TSS shot from supplier first, who will also provide you with his "tested" safe recipes (one of many), then go from there. You definitely do not want to source/buy hulls/wads,overshot cards/etc that you will not use.
If you want to get a few recipes, place a few small TSS orders with 2-3 suppliers and ask them to share their 12G 3" recipes with you. Ask for 1-2 recipes from each TSS supplier, so you will have a few recipes to choose from and higher likelihood of finding components for at least one recipe out of the group.
I have 12G supplies that I might would turn lose of, just need to do inventory and see what you need, versus what I have.
You are welcome to PM me with pointed questions if you like. I will gladly help you get started. I have helped a few on here and they are rolling strong at the moment.
Best of luck, hope you are able to get into it sooner, rather than later.