I'd go straight to the manufacturers. The two old time companies are American Target (Co) and National Target (MD).
I've always ordered from American Target in Colorado. They were always great to deal with;
http://www.americantargetcompany.com/high_power_rifle_targets.asp
http://www.nationaltarget.com/contact_us.php

The newer company back then (20 years ago) was Speedwell out of NJ. But I never had dealings with them, so can't tell you much.
http://speedwelltargets.com/cart/

Last edited by ChrisF; 02/28/15.