There used to be lots of 7.62x39, (though non reloadable)but no more, don't know why.
My understanding is Venezuela ordered millions of rounds of 7.62x39 from Russia, drying up the supply of cheap, surplus rounds for export to USA. Most mail-order places were out of it for a year or more. I could only find it at gun shows. It's just starting to show up again at on-line stores.
-Blast