I bought these 'Cut and come again' salad leaves from a supermarket for £1. Already germinated and half-grown, they looked ideal as started plants for my raised veg bed.
I carefully pulled them apart into several clumps, and planted them. They were watered regularly, and 10 days later: mini lettuces! Just cut a few leaves at a time, and serve with home-grown tomatoes.
Much cheaper than garden center starter plants, the quality was also better. If you do this every two weeks you'll have salad leaves into the autumn.