Well, I've been buying 6.8SPC off the shelf at Academy this year. And there's about 3 dozen makers of 6.8 out there, plus a large number of ammo retailers who sell direct over the internet and the truck delivers to your front door.
Of course, you have to plan ahead, and not suddenly remember you are out of ammo when picking up a gallon of milk and video on Friday date night. If BoxMart is your only source of ammo, that means generally buying stuff that's at least 20 years old since it's introduction. The firearms buyers at those places are the most hidebound conservatives ever seen, it's got to have a longstanding sales history with Elmer for a decade before they will make room in their limited locked case. What's in there is guaranteed profit makers, not leading edge alternative calbers for firearms they refuse to stock.
You shouldn't expect to see Aimpoints in clamshell packaging hanging on the hook, either.
It'll be years before .300 Whisper/BLK is stocked, too. It's been kicking around since the '80's when frustrated AR owners weren't allowed in main battle rifle matches because of a .30 cal restriction. What's interesting is the sudden push by Remington. In the last few years, the offered the 6.8SPC, the .30RAR, and now the .300 BLK Whisper copy.
Track record? The 6.8SPC took off and made #1 AR alternate caliber once it went viral with other companies willing to correct a small error. The .30RAR uses a special bolt and reputedly a different upper, it's not just another alternate, it's a different gun, like the AR10. Sales are less than staggering, nobody is buying into a single proprietary design, just like AR10's can't swap uppers, lowers, or some parts because different makers tweaked a non military non standardized design.
The .300? Found that cheap plinker ammo on the shelf yet? It's promised, but it's going to take a few years before it reaches being as available as 6.8. The focus on running it subsonic and suppressed is counterproductive, when the hunting market needs to see field performance. That's the MASS market that pays to get the stuff and makes it show up on the shelf.
Like at Academy. It may be 115gr OTP Remington green box, but it's there.