I asked the Glock representatives about that at the 2016 NRA Annual Meeting. The one rep I talked to stated that it would be more expensive to buy the slide alone rather than the entire pistol. He strongly suggested just buying the MOS pistol outright. Then again, he is a salesman.
I was told the same thing via a phone conversation with a Glock rep last year. I ended up buying a G34 MOS from The Cop Shop. I installed a Vortex Venom red dot with Dawson Precision MOS sights and I am extremely happy with the pistol!