The best way around this, is to grow your own.
Trust me, lettuce is probably one of the easiest of veg to grow from seed ( including those fancy looking ones with posh names-- they are not an exception ).You do not need a garden. You can even grow some varieties in pots on the windowsill.
The best ones to choose are the "Cut and Come again" varieties. These are a loose leaved type similar to what you find in the Supermarket salad packs, and as the name suggests, you chop off what you want and it will continue to grow. No chemicals, and a lot cheaper and fresher than the supermarket.
Another salad ingredient well worth growing, is good old fashioned Cress.
Often used these days as just a garnish, but fabulous in a sandwich ( butter not margarine though ). We grew it in Infant School on wet blotting paper. in a saucer. It will grow on anything as long as it is kept watered.
Happy Days.