The boards look like coroplast or whatever it's called. It's plastic corrugated to look like cardboard basically. Sign stores usually have it in 4' x 8' sheets, my local one will cut them down to 2' x 4' (much, much easier to wrangle) without charge, but other places may charge.
Free or low cost source is old political campaign signs. A lot of them are using these now, and post election, you can often get them free. A good scrub and they are good to go. I've not noticed any color transfer but the longest I've held anything on a sign was about 9 months, so I've not tried long term storage.
To cut - T-square and a sharp utility knife have worked for me. It's SLOW cutting so the lines are across, but otherwise it warps when you wrap around. A jigsaw or sawzall might work, not sure.