Save growing your own lettuce plants from seed, a quick way to get going is to nip to Lidl and buy a tray of their 'Living salad'. It retails at 99p and easily contains 30 - 50 plants. They are designed to be eaten as they are, but I haven't the heart to eat plants so young and deem it a waste. So all I do is prepare a bed for them and plant them out. There are 3-4 varieties in the tray and lasted me all through the summer ( watch out for the slugs/snails though). They are grown in Yorkshire and are a smidgen of the price charged in garden centres. Maybe not as cheap as growing from seed, but a damn good short-cut !
PS - I do the same with live herbs by buying chives, parsley and basil, just loosen them up a bit and stick them in a sunny bed, they really get established pretty quickly. Just try and buy them when they are fresh in as they can get a bit 'leggy'.