Find a local supplier of 1/8” birch ply and mdf. Look for lumber suppliers, avoid big box stores like michaels and wal mart (they have bb ply but it’ll be 5-10x as expensive).
Cardboard is nice to have, start scoping out a plentiful box source. Try to choose a box you can get more of (amazon boxes are good like this for me) because that way you’ll always know the settings — Not all cardboard is the same.
Speaking of cardboard always be careful with it especially with intricate cuts. It can and will ignite.
Other than that maybe get some leather scraps at a local shop. Stay with vegetable tanned, chrome tanned is super bad for you.
Splurge on one or two exotic hardwoods. I like green valley (gvwp.net) but west penn hardwoods and ocooch hardwoods are solid suppliers too. Wenge, padauk, yellowheart, Purple Heart are all solid materials and will amaze people who haven’t seen them before. Gvwp sells “multipaks” of 3 different woods, it’s a good way to get your hands on some different types, albeit at 3” widths. I prefer their packs of 6” pieces, but for inlay 3” is plenty in most cases.
Anyway it’s a huge topic. Beads. Rocks (try worry stones, especially in dark colors like jet). Shot glasses. Grains of rice. Skulls. If it fits and isn’t loaded with chlorides or other nasties, try it out!