Plain is a flavor

Plain yogurt can be full fat, low fat or fat free.
What I've found, sometimes it's hard to find single servings of plain, non fat yogurt. I've had to buy it in bigger containers and then portion it out at home. Stonybrook and Horizons are 2 of my favorite brands. You can always make a local request at your favorite grocery store for them to carry what you want (that's how I got the small size La Tortilla Factory tortillas at my neighborhood store - I asked!)
Please don't get so caught up in perfection. Life is messy and complicated! It is just my opinion, but a certain amount of flexibility is KEY for long term weight loss
Good luck!