Chef John's Buttermilk Pie
You can use 1/2 of my Buttercrust Pastry Dough for this recipe--it would be perfect!
I've never had much trouble finding buttermilk, even in regular, non-fancy grocery stores, but depending on where you live in the world, apparently that's not the case. There are many "hacks" for making a substitute, usually using milk and lemon juice, or vinegar. I'll let you Google those at your leisure, but I'd be more inclined to try some yogurt, thinned out with some milk to be closer to the tanginess of buttermilk.
Once your pie is made, it can be served "as is," or topped with seasonal fruit. I went with raspberries, mostly for the pictures, but if you were going to do this for the holidays, some persimmons, and/or pomegranate seeds would also be very nice.