if there's an 'ethnic' section in the produce department, you might find all sorts of things that will take
root and grow.
a bulk foods aisle could also offer plenty of options: alfalfa, pumpkin, wheat and other grains, quinoa, sprouting mixes of brassicas, sunflowers, peanuts, chia, and others. both places that cater to unwashed hippies and wealthy foodies are likely to have more promise than a thriftway (do those still exist?).
then there are seeds from fruit. not great for groundcover, but could grow plenty of fuel, fiber, and mulch. stone fruits, pomes, citrus, cucurbits, nightshades (tomatillos will form a mat). fresh mint and rosemary are easily rooted. thymes and oregano are marginally more difficult. potatoes, sweet potatoes, yams will grow if they haven't been treated to prevent it.