Steel Threaded Rod 3/8in
Threaded Rod (Allthread) Coarse Zinc 3/8"-16
3/8 in. x 10 ft. Galvanized Threaded Electrical Support Rod
Don't need galvanized, ie zinc plated is fine.
http://www.homedepot.com/p/Superstrut-3-8-in-x-10-ft-Galvanized-Threaded-Electrical-Support-Rod-ZR1038/100207537 $6.52 for 10 feet
http://www.dunnlumber.com/Store/ProductDetail.aspx?pg=0&pid=14050 $7.79 for 6 feet.