I need to whittle out a transom piece before I cut it out of my plywood. Problem is I don't know where I can find a 66"x30" piece of cardboard. I thought about going to Home Depot or Lowe's or something and asking if they might have any from a fridge install or something but thought I'd check in here first.
Stop by your local bike shop. Bikes come in big cardboard boxes.
Appliance shops.
If you are willing to spend a little money, Amazon, HD and Lowes all sell Ram Board. It's 38" wide on a 50' roll.
Local motorcycle dealership. Most of our bikes, quads and SxS's were shipped in metal crates wrapped in cardboard. We filled dumpsters daily with it.
Local factory and get a couple slip sheets
Coroplast Political yard signs? We just had a local election here.
For big sheets, we have a local paper products company that sells 4x8 sheets. Off the top of my head, they are $3-$5 a sheet.
Thanks guys, I'll go wandering around on my lunch hour.
Bodyshops and auto parts stores too.
Hoods come in big boxes.
My work. Faces of our condensers have 3-4 4x8 sheets per side.
Furniture stores are a good source too. I just threw away the boxes my grandsons bedroom set came in, the largest was 3 x 4 x 6.
I keep way too many boxes for stuff like this. Some day I need to work on better ways to store/organize them.
Hospitals can be a good shot, if you can charm your way to speaking with their supply and shipping departments. Just know some of their cardboard is also for specialty stuff, so sometimes it's super thick or unusual.
Costco is another great resource.
I just tape small pieces of cardboard from amazon together...