If you haven't seen the announcement yet, my lumber & gypsum loads are now listed at Wig-Wag-Trains.com. They are listed under the brand "Leader Loads". So the next time you place an order with them, throw in some lumber or gypsum loads in either the 73' or 60' version. Of course you can always buy them direct from me, and I try to keep some inventory on Ebay at all times too.
I haven't expanded my 73' load offering in a while, however, I am now making every version in the 60' length for Micro Trains centerbeam flatcars. I don't know of any other source for a drop in modern lumber load for the MT cars.