Wolverine, Wanstalls, Budget Shooter Supply all carry 300BLK/Whisper ammo. 300BLK ammo is a bit pricey and probably more for the reloader if you plan on shooting lots of it unless you have deep pockets. Save your brass if you ever change your mind and start reloading.
Watch out for the 208gr subsonic Hornady ammo. Works great in a bolt gun but does not always cycle in semi-autos.