I went to the local comic store and got the comic boards but you can also go to any fabric store that is getting ready to dispose of the fabric boards and also Walmart. First thing I do is head back to the fabric dept and grab what is there. it's free. If you have any corrugated boxes that you're about to recycle, use them. You can cut them down to size you want. Use a box cutter or an Exacto knife. Have an old photo frame that you can't use the back any more use that. Boxes that hold your reems of paper for your computer. If you're not storing anything in them use those. My DH now cuts up the boxes that anything comes in to his office rather than throw in trash. he knows just about what size I want 8x10. Tell your neighbors you'll take their boxes if they don't want them. Some gift boxes that have out worn their use. If it's firm enough and you can cut to size use it. No one will know how ugly it is until you are almost done with the fabric.