Places like Home of Economy, or Tractor Supply. When they get in a shipment of trailers they are packed with rectangle styrofoam blocks. I built a backstop against my house in my yard a few years ago and it's still holding up. I used t posts to wedge them against the house and used ratchet straps to hold it all together nice and tight.
Just recently, I built another for indoor use since I have access to a heated warehouse where I can shoot to about 75-80 yds. I made it pretty big as my boss wouldn't appreciate holes in the warehouse. Plywood frame built onto a pallet for easy moving with one of our forklifts. In the Spring and Summer I do most all my shooting behind one of our warehouses so this will get moved out there then. I also bought a bigger bag target for when I shoot at longer ranges. I got sick of losing arrows!!
There is two layers of blocks behind that cardboard, and I used cardboard on the sides to wedge things tight. Ain't pretty, but it'll work.