The first page states in RED, "All thicknesses 4" wide or greater are either out of stock or will be until furtrher notice. That's not a good sign.
There was a riff on that theme on the Lone Star site -- balsa is getting pretty popular in industry for soundproof doors and Corvette floor boards (they stick a big chunk of balsa between the fiberglass layers for soundproofing). Model airplane balsa is a very small part of the overall market, so they get low priority.
Hence, sometimes your supplier will run out, and you'll have to wait until they get another container full in before they'll have balsa to cut.
Having said that, finding a supplier with stock that's consistently in will always be better.