You don't buy green manure, Eileen, you grow it. You buy seeds of green manure plants, sow them in empty space in the veg garden and then dig the green plants in later so that they rot down in the soil.

You can get the seeds from Thompson & Morgan or Chase organic.

Make sure you buy a species for winter growth eg red or crimson clover, vetch, mustard, italian rye grass. What ever is appropriate to your locale.

Field beans are good for winter growth too