Hello,
For barbed wire I used regular old sewing thread. It was a brown color so it looked rusted. All I did was stretch the tread out from post to post, or where ever you want the wire to run. Then take shorter lengths and tie them in a knot around the long wire. Tie them every 1/8 inch or so, and snip both ends of the shorter knot leaving enough to give the apprearance of a barb. This cost me $0, and if you dont like it you can buy a $8 photo etch. Not sure if there is a real difference from one manufactor or another. <<Optional>> One more thing the thread may need elmores (spellchek) glue or wax, rubbed down it to keep it from looking fuzzy. Then do the knots.
Good luck, I'd like to see the finished pics.