Cheap, Convenient Target Stands:
Take two or more wooden 2x2 posts about 3 or more feet long and cut one end at an angle to make a spike.
Drive them in the ground next to each other about two or three feet apart. They need to go into the dirt at least a foot or two.
Wrap chicken wire around them so it covers the space between them. You could staple it to the posts for extra strength, but I found that not to be necessary.
Use clothes pins to hold targets to the chicken wire at four corners. It has to get pretty darn windy to blow the targets around!
This idea could be improvised to make mobile target stands by nailing the posts to a large wooden base from underneath. The base would have to be large enough that wind wouldn't blow it over.