I just bought 3 packs of the Ateco Heating cores from pastry chef. com. They are $3.99 and come with in 4 in a pack. I haven't used them yet but I always used the Wilton flower nails as make shift cores so I expect these to work. These are stainless steel so they shouldn't rust like the Wilton nails (I know it is because they aren't meant to be baked
Each package comes with a helpful diagram to tell you how many cores to use for various pan sizes.
Edited to say: OP, you were right-you probably read about them in a post here because I read about them on CC.
I hope this helps!!