Didn’t read the previous answers; apologies if I repeat anything.
Do you have the option to grow some of your own food? If so, do it. And learn how to can food, also. It’s not hard, I promise. A solid garden can take care of you all year round if you know what you’re doing (I’m actually currently giving away some canned goods, as the harvest last year left me with more than I apparently needed). Plus, you can control pesticides, etc. and allowance for variation (e.g, stores won’t sell “ugly” looking species).
Check your area for local food and compare prices. Depending on your location, you might have neighbors that sell meat, dairy, or produce, and often times you can get much better quality and a better price than store-bought. Buying meat locally makes me feel better on a moral level, anyways, just to know that the livestock is properly cared for and not overcrowded (and all the other issues that rise from factory farming).
Buy what you can at “discount” stores. My go-to is Aldi. The ones in the area I’m currently in don’t offer the best produce, but I still buy their dairy, basic home and bath products, grains, canned goods, and frozen foods. If you can fit it in your freezer, buy it in bulk. Keep in mind that frozen fruits are great for smoothies [you can also freeze any greens (spinach, kale, arugula, etc) that you might add to your smoothies].
Lastly, coupons. Although, I don’t actually have any experience in this department.
The reality is that to live frugally (and sustainably), you have to rely a lot on “the old ways,” which often requires a little bit of effort that most of us aren’t used to anymore. Personally, the pros outweigh the cons tenfold.