Perhaps its a convenient thing here in Wisconsin, I don't know..being a logging wooded area...but, I go to lumberyard suppliers....or places like Menards. You can purchase a 4' x 8' sheet, 1/8th inche for about $13....and a 1/4" for about $17.
I also look for 4'x 4' cuts of birch and cherry veneer...which is quite light for making many panels intended for travel, and cut accordingly.
One has to factor in that wrapping canvas around the edges will widen and length boards, so one might want to cut their sizes about 1/8th" shorter. Or, simply use a razor and clean cut the canvas to the edge.
"Painting is easy when you don't know how, but very difficult when you do!" Edgar Degas