Finding those stock sets is almost impossible. In the U.S.A., they were originally used on the Type 56S-3, which is a rare and collectible early AK import. Since those are valuable rifles, most people wouldn't alter them by removing and selling the stock set.
I did see a complete set sell on Gunbroker recently, and it went for somewhere around $800.00, if I recall correctly. The pistol grips are available, but expensive. The upper and lower handguards are rare, and very expensive. The butt stock is simply not available, at any price. Over the past 10 years or so, I've seen one (one!!) Chinese AK red fiberglass butt stock for sale by itself. It was part of a "package deal" on Gunbroker, and I don't even remember what it sold for. I'm sure that the buyer was thrilled to get it.