Fish - overfishing is obviously bad for the environment
Avocados and nuts - out of season in North America right now, fossil fuels used for transport, drought in areas producing nuts in North America also bad
Olive oil production, also bad for the environment
Leafy greens I believe can be produced locally (Canada), not necessarily affordably, although frozen options do exist
Most affordable and enviromentally friendly will usually be seasonal local foods. Ofc this then limits your options a lot, so maybe try to look for these options first, and then grab anything else to fill the rest of your diet. Eating more veggie options can also have a huge influence on environmental sustainability and can open your palet to some awesome new foods. Just switching one meal a week makes a huge difference. However, some veggie options can be a little more expensive, so look around and find some stuff you like! Omega fats can be found in a lot of seeds and nuts, including chia and hemp seeds which are super easy to chuck in your meals for extra crunch.As long as you’re regularly consuming around 5 portions of fruit and veg you’ll probably reach your potassium requirements very easily.
Not all nuts are not out of season in my part of America right now. My yard is full of pecans and only a month ago I had to be careful on my walks because of the orchard trucks hauling loads of them to the shelling plant. My neighbor still has black walnuts everywhere. Peanuts may be out of season, and hazels and almonds don’t grow here, so they may be out too. I buy Texas grown olive oil so the environmental impacts are monitored pretty closely.
For one, this would probably involve buying a lot of food in the spring and summer months at things like farmers markets or just straight from farmers you trust and then preserving/canning what you can’t eat. Not much grows in the winter, just the way it is