Either ‘Foamcore’ or ‘Gator

Either ‘Foamcore’ or ‘Gator board’ would work for making your oversized dominos. Foamcore is less expensive, but is also more flexible at larger sizes. Gator board is very rigid, yet lightweight, but is also more costly. Find an art framing shop near you – they should at least have foamcore in stock as it is the industry standard for mounting photographs. Good luck!