I don’t buy lettuce. Instead I grow my own, inside in winter, and outside in summer. I have always grown lettuce, even when I had a flat without a garden. Lettuce seeds are so cheap and a packet will give me leaves to put into a salad for a year. Admittedly, I do buy different seeds for winter as my house is so cold. I buy a Russian variety of seeds that grows in cold weather. I usually get these from Real Seeds. They offer low postage and cheaper seeds to those on a low wage or income. Growing lettuce saves me so much money and I have access to fresh food all year.
The picture of the lettuces above were sown 6 weeks ago and grew in the conservatory. Earlier in winter I grow lettuce on window sills where it is warmer. My lettuce are started off in an old grape tub and transplanted into Christmas sweet tubs, with holes drilled in the bottom. I sow lettuce every 6 to 8 weeks to keep a constant supply. No warm mats, lights, or special equipment is needed.
Outside I tend to grow lettuce in old catering mushroom trays on top of a table to reduce the chance of slug attacks. I also plant them in a massive tray on a stand at the front of the house. This is North facing and so the lettuce are less prone to bolt with it being cooler. They last a lot longer there. I hardly ever pick a whole lettuce. I tend to just pick the outer big leaves, and they keep producing and growing. These should last 6 to 8 weeks in these tubs. I always make sure that I leave 3 leaves on each individual plant. Who needs supermarkets?
Here are some ideas of other things that you can use in a salad before the garden starts producing properly. Do you add anything different?