Hi Turbo Gardener, welcome! Here are links to some lettuce varieties that are root aphid resistant. I hope these are useful
Marshalls have Lakeland lettuce seeds which are resistant to root aphid.
Lakeland Lettuce Seeds | Spring Cropping Lettuce Seeds
Little Gem is resistant and available at a number of seed merchants including Suttons.
Suttons: Vegetable Seeds, Lettuce Little Gem Seeds
Beatrice and Avoncrisp lettuce varieties are both resistant and available at Seeds By Size.
SEEDS-BY-SIZE LETTUCE SEEDS 2009/2010 LIST