Here's the metal shim I use: Big Shot Adaptor Plate - Cheery Lynn Designs
When it first came out, it was called a "Cuttlehug" because it was made for the Cuttlebug. Now they have one a bit larger for the Big Shot, but my Cuttlehug works just fine in my Big Shot (or at least it will until I need to cut something wider than 6").
Angie, I assume you meant "shimming" - it's adding layers to your diecut sandwich before running it through your machine. Sometimes you need as little as a sheet of copy paper; sometimes you need a several sheets or a piece of cardstock or cardboard.
I started out with a Cuttlebug; I've never broken a B plate, but did break a "C". Can't remember what I was trying to emboss, but it wasn't working; so I turned the entire sandwich around leaving the A plate on the bottom, then the B, embossing mat, paper and my die, with the C on top. CRACK! So glad I had an extra on hand.