I feel your pain. Sour cherries are hard to find even when they are fresh and in season. The vast majority go to processing.
One product that I *think* is distributed nationwide is Oregon Fruit. They have pie cherries canned in water.
I know it isn't cheap but even with cherry country near me (Washington/Oregon) sour cherries are scarce. It's too bad because sour cherries have a lovely flavor. I have seen 100% tart cherry juice bottled in the stores too, if that helps.
You would think you would be able to find frozen sour cherries in large bags because most of them are processed in some fashion. But I have yet to find them.
Canned cherries might not be ideal for you but I did can my own sour cherries this year and (in water) and the flavor was pretty well intact when I decanted them. And I have used that particular brand of cherries in pies before and they were fine. And at least you don't have to pit them. Pitting a large number of cherries is a time consuming pain in the neck.
Many thanks, Nick Z. FYI there is a tool that easily pits cherries as it extends a small bolt that pushes the pit through from the middle to the outside of the fruit. Presumably, pitting cherries in that way allows the alcohol to extract more flavor from the fruit.
I used a device that pitted seven cherries at a time by pushing the pit out through the bottom of the cherry. Even with that it was tedious. I suspect the tool was designed with larger sweet cherries in mind.
At Costco? Really? I went there the other day and didn't see them. Is that seasonal? Because I've wanted more pie cherries. I actually regret canning my cherries now because I think it harmed the flavor somewhat. I wish I had frozen them. If I could get some frozen ones that would be great. They will mostly go into jam but I'd love to have some to mix into ciders as well.