I've never frozen buttermilk, so can't help you on that one.
I've used low fat sour cream or plain yohurt as a substitute before with excellent results.
You can also get powdered buttermilk, I think. I've never tried it myself, but my MIL uses it. That would prevent waste - you could just reconstitute the amount you need, and the rest would keep without issues. But, like I said, I haven't tried it myself.
All the best!