Greek yogurt is super easy (and cheap) to make yourself if you have an instant pot:

Put 3L of milk and some starter from your last yogurt batch in the instant pot and press the "yogurt" button. Set an alarm for 10h.

Pour the yogurt into a strainer lined with a cheese cloth, and a capture vessel underneath for the whey, then put it in the fridge overnight.

You now have 1.5L of Greek yogurt that tastes head and shoulders better than anything you'd get at the supermarket. Takes me about a week to eat it all.

If you're worried about a spoiled batch ruining your next starter, you can take the whey from the straining step, pour it into an ice cube tray, and keep it in the freezer. 2 cubes is plenty for 3L of milk and can keep for 6 months.