As Dan said Masonite is one of those brand names that became a generic term like Kleenex or Jello. What many people call Masonite is tempered hardboard, also known as tempered HDF or tempered high density fiberboard. It is a pressed wood product. The tempering process provides a hard smooth surface on one side. It is the same material often used to make peg boards for hanging tools. Generally sold in hardware stores in 1/8" (3mm) or 1/4" (6mm) thickness, and 2x2, 2x4 or 4x8 foot sheets. I was going to make myself a drawing board, but then found Hobby Lobby sells a 23x26" drawing board for $12, which was less than the material cost to do it myself. They also sell a smaller 13x17" for $6.50. I'm sure you could find something better, but this one works fine. It is a hard surface, the clips and rubber band hold the paper, not much more I could ask of it for $12.

I'll second the recommendation to try your art supply store. They may even be able to cut one to a particular size for you. If that fails, your hardware/humber store ought to be able to cut you a thin sheet of particle board to whichever size you want. In that case, you would probably need to invest in some sandpaper, but you need that anyway for sharpening your pencils.

Sup Tim. You looking for a drawing board, like to hold your paper as you draw? I find thin masonite kind of flimsy for using as a drawing board, personally, but I can see the appeal. Cheap! The hardware store typically has it listed as hardboard or tempered hardboard, so... Huh... Don't think the Masonite company is doing anything but doors these days, but I can't imagine Lowes or whoever doesn't have just boards of it?? Sounds like the people in your stores don't know what they're talking about haha. Commercial drawing boards are fine, though. I use a Helix brand 18x24 board myself with a couple big ass binder clips. It's basically just a piece of plywood with some metal edges, though. I'd see if anyone around you has any off-cut single-ply hardwood plywood. Sand it down, seal it with some polyurethane, and it'll last forever. You could get some MDF, too, but don't get it wet. The giant clipboards at Michaels, Hobby Lobby, Craft Warehouse, etc. are okay, too, as long as you don't beat up on them. Huh...

Check your local art stores instead. They will be in the drawing board section.