I buy a lot of seeds from Lidl, particularly peas and beans. They are currently selling off stock at 20p if you’re quick. I have also seen onion sets in there, but usually only for a week. I tend to use B&Q for stuff like onion sets and potatoes or a local garden centre, but I need to make a special trip for the garden centre and always seem to just miss the potatoes/onions whereas I’m a regular in B&Q.