Green Manures

I've always used greenmanure.co.uk for mine.  You can buy the seeds by the gram and they come nicely packaged in big paper bags.  It's a lot cheaper than the stuff you can buy at garden centres and they have a nice variety to choose from.  Their 'winter mix' is a good start, as long as you make sure it fits in with your crop rotation (e.g in some places you may not want a nitrogen fixing variety)