I just bought the Fiskars Fuse, which is also similar to Big Shot & Go. I have bought a variety of shims for it and I just sort of mix & match until something fits.
For shims you can use pretty much anything solid that won't compress under pressure. Most of the ones I've purchased have been plexiglass or dense plastic, but I've also added chipboard to bring things to full height and it works great. I've read of other people cutting the sides off of cereal boxes and using those as shims - one or many, to suit.