Yes, of course you can buy 1 X12 lumber at a lumber yard!! I wouldn't use it, however, because it will warp and the board will become dish shapped and hold water!! I use greentreated 3/4" plywood so it doesn't warp up and create an uneven surface. On top of that, I use a 3/4" rubber mat cut to fit the dimensions of the chute so it doesn't slip around. We had some old "runner"mats out of a horsetrailer that worked really well. This is a heavy system, but usually around the farm that's better anyway--especially with new calves, etc. We use a lighter chute for shows, etc. but I really like a heavier steel chute in our barn for everyday use.
We even use our Foremost working chute to wash calves. WE have it set up on a sloped concrete floor so we just lead the calf in, put his head in the headgate. Then, I just put a couple of 1 1/4" round pipes vertically in the holes, open up the side doors, and go to washing. I never have to worry about a calf getting a side rail off and tipping the chute over because its so much heavier than a grooming chute. Also, we don't have to do the "Wash Waltz" with the calf swinging their butt back and forth from one side to the other. Once we've washed them a few times this way they are usually calm enough to just tie them to a fencerail to wash them. Usually, though it works so well where we have it set up that we just keep using the working chute.