We have our regular go to choices from our weekly grocery trip. We mainly buy what we like and know we'll consume before it goes bad. When cooking we prioritize cooking older stuff so it doesn't rot. In the winter we buy more frozen vegetables. I also always have legumes that I've cooked myself ready to go in our freezer.
Beyond that, Subway veggie subs are worth looking into. Really. You won't get a healthier, cheaper meal almost anywhere. They updated their menu recently and now have an Avocado sub that is fantastic.
I have a standard routine on the weekdays for dinner,
1. salad with some protein and carbs(potatoes mostly). Salad always has a standard base of vegetables like lettuce, carrot, cucumber and sweet corn, with dressing (apple cider vinegar or yoghurt) and salt. Sometime I add the salad dressing mixes or raw mangoes or green apples just to keep it interesting. It's always 60% salad, 20% protein and 20% carbs for the meal.
2. A semi-gravy vegetable stir fry of mostly carrots, bell peppers and mushrooms with a rotating list of vegetables such as beetroot, beans, brussels sprout or broccoli. I cut them up in small chunks and roast them in the oven mixed with some olive oil and salt. Then I prepare a sauce (mostly experimental) and add the cooked vegetables into it and saute for a bit. I sometimes add meat or meat stock and have portion of it as a full meal. In case I am really hungry, I mix in some carbs in the form of brown rice or wheat tortilla.
I alternate between these 2 every night and I cook enough for it to be carried over as the next day's lunch.
Even if I go outside this routine(on weekends or cravings for fast food), I try to ensure that the meal I have at home is at least 50-60% vegetables, and salads tend to be the most easiest to make up for this balance.
During about half of the year, local farmers sell their produce from stands and at farmer's markets. When possible, that's where I buy them. The rest of the time, I buy them at the supermarket.
I buy most vegetables (exceptions include things like squash that have a long shelf life) a day or two before I prepare them.