Where to buy SMALL pieces of foam

[RobertInOntario]

Home Depot (or any other big box home improvement store) might have broken pieces of extruded (pink or blue) foam. I bought 2 sheets of 2x8 that had been crushed on the corner and the shiplap broken for $10, instead of $20 each. Scraps might be had for nothing. Make them an offer - that's what I did.
I would not use the white "beadboard" unless it is supported by something. It's not very strong.
Andrew

Thanks, Andrew. I'll check HD again -- there's one only a few minutes' walk from where I live!

Yes, the white foam is weaker -- the rest of the layout uses the stronger green (environmental) foam -- but I only used it to increase the layout's width slightly. Hopefully, it will still hold up OK!

Rob