Gems — how to buy expensive stones

Anecdote:  in late summer I got a call for a 4+ carat lagoon tourmaline of a very specific cut and color, not too dark, etc etc.  Unsurprisingly, the potential buyer couldn't find such a stone anywhere.  I asked the buyer if they were serious about the purchase because it would be expensive and hard to find.  I come across about 100 lagoon tourmalines in that size a year at best and I know who imports them and the source.  I know Tiffany likes those stones too and they pay good money for them.  Out of those hundred or so stones,...