I have bought a lot of nice cabs and other stones from here;
http://www.manchesterminerals.co.uk/trade/index.html ,it's worth registering with them also. They used to be known as Hilton Bros. or Hilton Jewels. They can be very helpful if you want specials. They will email you photos of items from their special stones stock if you need them. I bought a batch of reasonably priced square facetted emeralds from them for a restoration job I was working on a while back, and all of the cabs used in my daughter's rings came from them.
James