It is few and far between right now. I am a dealer for ADG and have some on order, but it is looking like February before dealers will be seeing the next run. If you don’t have any luck finding any shoot me a PM and I can put you on my list for back orders.
To your original question, Hornady also makes 300 PRC, again hard to find right now. Lapua is slated to start offering it, but it will be after the first of the year. RCC also offers it as well, but you will pay a large premium.