You can either make up some new ones from metal, just make sure they are similar to the original ones...
or
You can buy a new cross sill from several suppliers....
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=20
Look for item listed as 5 under Wood Bed....
Other vendors have them as well, similar prices....or maybe someone has a second hand one they could sell ya...
