In the past, Rod at Orca Boats has been willing to sell blocks of the grey minicell foam. He uses it in the construction of his wooden canoes and kayaks, and usually has it on hand. I haven’t seen him in a while, so there could be changes. Last time I visited, he was living in Coquitlam.
Website address: https://orcaboats.ca/
If you live near Burnaby, I also have a box of scraps that might contain something the right size. You’re welcome to look through it. As long as you’re not trying to walk off with my foam kayak seat, you can have a chunk for free. (I have no idea why I keep all these little pieces. Hoarding is such a disease!)
Cheers, Reed Clarke