My club uses corrugated cardboard fastened to 2x4 uprights with wood strips & screws. Very cheap, gives good bullet holes, opaque, and holds adhesive well. We paste our targets on it with premixed wallpaper paste purchased in 5 gal buckets.
This place has them
http://www.boxforless.com/categories/Corrugated-Cardboard-Sheets/ but we buy 96x96 sheets by the pallet from a local supplier.
The reason to prefer adhesive over staples is that a bullet can send steel fragments flying if it hits a staple. Doesn't happen often, but you don't want to be the next.
