It’s kinda like MDF, just not as tightly compressed. Same stuff that peg board is made from I believe. It’s often used on the back of knock together furniture to stiffen the structure. Ask for hardboard at your local building supply store and they should know what you’re after.
Aside from being flexible as hell (especially in a full 4x8 sheet), it’s not too bad to work with. It’ll be as straight as what you fasten it to. 