Plastic Grocery Bags. Most regions require recycling of these, so most people save them for pick up. Your neighbors probably have a bunch. You can also call the recycling center and get them there. Grocery bags make a lightweight filler and they don't soak up water.
Pallet Wrap. Available anywhere pallets are unloaded, like supermarkets and warehouses. Lightweight and waterproof.
Plastic Wrap from Baled Goods. Probably the easiest to get are the plastic wraps from baled cellulose blown insulation (don't use those from fiberglass insulation). Call a company that does blown insulation (they're everywhere) and ask them to save the wrappings by stuffing them into garbage bags for you and you'll pick them up. One large garbage bag stuffed full will do one target. They have to get rid of them one way or another. Lightweight and waterproof.
Polyfill: From old pillows, duvets, sleeping bags, etc. You don't have to take the stuffing out of them if you don't want to, just make sure there is enough thickness to the target, and no gaps in the filling.
Cloth: Old clothes, sheets, curtains, rags of any kind. Excellent arrow stopper, but heavy and can soak up water and get even heavier.
NOTE: You can mix the above fillers, just make sure the bags are properly compacted with no voids and that the corners are filled. There are lots of similar materials you can use too. Pack the target down lightly as you fill it so it is nice and fat, like the one at left.
Closing the Bag
Click on the picture at left for the slideshow
It's easy to make a range target face from a Treeline Target bag by simply splitting the bag. Start by carefully undoing the sewn corners, being careful not to cut the bag. Next you have to undo the seam at the top of the target. There's an easy way and a hard way. Here's the easy way: One side of the seam has a single line stitch, the other side has a chain stitch. Lay the target on a table with the seam at the top and the line stitch side facing you. Snip a few of the line stitches at the far right of the seam. Lift the corresponding chain stitches on the opposite side and then continue pulling on both sides at the same time. The entire seam should unravel easily.
If you're using a DIY target frame, once you have stapled the face to the target frame, coat the attachment surface with glue (contact cement will do) and let it harden before you put the finish strips over it. This will help prevent the target material from pulling off the frame, but when it's time to replace the face it will pull off without too much effort.
To see an example of a DIY range target frame go to DIY: Lifetime Archery Target over at archeryreport.com.
You can make a simple and effective backstop with a wood frame and a piece of ordinary carpet. The one at left is 5 feet by 5 feet, and made from 2x4's and plywood gussets. The piece of carpet is 4 feet by 10 feet. If you don't have any carpet, call a carpet installer and ask him to save you a piece of used carpet the size you want. The carpet is draped over the top 2x4, so you have two layers to stop arrows. The arrows are a little hard to remove, but you shouldn't be hitting there anyway.