Wild Sorrel Rumex acetosa can be found growing in many types of grassland, especially if the soil is acidic. The leaves resemble arrow heads which are quite often clinging closely to the main stalk. It's probably worth a walk around your garden to see if you have some as it quite often grows there, if you don't cut your grass too often. Failing that, you can quite easily obtain the seeds, so why not grow your own?
200g sorrel leaves
100ml of double cream
Wash the leaves and remove any thick stalks. Cook gently in butter until dark and wilted. Sorrel does turn a rather off putting brown when it gets warm, but this is fine. Stir in the cream a little at a time, tasting until the desired sharpness of flavour is reached. Season with salt and pepper to taste.