Artisan Bread Shop with Sourdough Starter to buy - Central London?
Ebay seems the easiest,
but Poilaine just off Victoria definately bakes on the premises judging by the heady, intoxicating aromas in the shop. Their pastries are great too. St johns, maltby street, sat mornings on the other side of London is a close second for my favourite bakery, especially for doughnuts.
Each are worth a visit in their own right, but phone ahead for the starter.
I can't help with what bakers might be willing to give you a gob of starter....but I know you can buy starters of various specified provenance on Ebay.
I have recently embarked on this journey and the best (unsolicited ;) advice I can give is to buy the "Tartine Bread" book. I have followed a few books in my search for a home made sourdough loaf that rocks....and the country bread from Tartine is amazing....couldn't believe it was possible from my oven.
A key tip he gives is to intially bake loaves in a dutch oven, which proves vital in trapping steam (otherwise impossible in a home oven as it vents steam quicker than you can generate it with ice cubes etc) ....thus replicating the steam injection bakers oven and forming the basis for that illusive dark glassy crust to die for.
Having tried various sources of starter....I would urge you just to go make your own.Coulnd't be easier, and the magic happens (imo) in the flour quality/hyrdation/handling techinique....rather than the eons-old starter.
Hth....and sorry if I repeated stuff you knew! :)