Buy?
Dude, go down to Home Depot or Lowes and buy some 1 x 2 strapping and a box of cheap 1.5" wood screws. Use the cheapest strapping they have. You'll need a little over two 10' lengths for each 5' high stand. You do have a screwdriver and a saw right? (If not then turn in your "man-card"
)
Step 1, build the actual holder: Cut 2 uprights whatever height you want your target at. Cut 2 crosspieces the width of the targets. Screw the crosspieces to the uprights.
Step 2, build the bases: (You're making an H-shaped piece that the holder plugs into.) Cut 4 more crosspieces. Screw two together aligned in parallel and spaced apart by a couple of scraps of 1x2. Screw the remaining 2 onto the ends perpendicular to the ends. If you space the scraps right you have a couple of pockets exactly the end of 1x2 to plug the holder into.
Time to build the first 1: 30 minutes while you figure out what the hell I'm talking about (wish I had pic's). Time to build more: 5 minutes per (with an electric screwdriver).
ETA: if you're really cheap you can skip the crosspieces on the holder part and just use a cardboard backer or cardboard targets.
The bases last forever if you don't throw them around. Replace the shot-up holders as required. Apart everything lies flat in your trunk or trunk.