ok, so not any - WF and BGD have their diamonds in their offices. But can Yoni in NYC who I bought from before source a James Allen diamond? I'm feeling a bit underwhelmed at the 2.6-3 carat selection options.
Just ask. I always ask whomever if they can source by the GIA report number. Often times, yes. Sometimes no, since they don't deal with the diamond broker.
Depends on where the diamond is located and who owns it. I worked with IDJ and scoured all virtual inventory retailers in search and would then ask them what they could get. If the stone was in NY, it was easy for them to bring in. There were overseas stones that were owned by someone who had a US office and were willing to send the stone over without first purchasing it (although this is not as common). Remember that vendors have access to way more inventory than we could search for...
I also worked with a B&M to source stones I found off other online retailers. I probably suggested 12-13 different stones. They were able to get me pics of 7-8 of them to check for clarity and type of inclusion and level of eye-cleanness. Out of the ones that were sufficiently eye-clean we brought in two the first round. I compared those with 3 others they had in stock. Criteria was close to 3, H, VS2. It was not easy. I hope your vendor will work with you and bring in the stones you request. But my vendor was nice to pull pics of stones so we could eliminate ones that had more visible inclusions under the table, etc. If these two didn't work, I would have had to go back to the drawing board.