I would agree with Scotzm that Bill Barrell (
http://www.barrell.co.uk/) is one of the best in the business.
Other places to buy would be from Andrew Chappel
http://www.qvpennies.comDoug MGill
http://www.mypennyblacks.comMartin Jenkinson
http://www.sturstamps.com/index.htmRobin Cassel
http://www.mulreadyphilatelics.co.ukI have no relation to any of these, I have bought from them all, they all trade with a very high level of integrity and one can be assured that a 1d black thought from any of these gentlemen will be an excellent example. Not necessarily a great buy, but rather a fine stamp at a fair price.
Just my 2d.
Larry