Hi Blair. I am currently using two vendors for 99% of my herbs:
San Francisco Herb Co -- www.sfherb.com
Mountain Rose Herbs -- www.mountainroseherbs.com
There are lots more, but for accessibility of information, origin and lot tracking, well-stocked inventory and price I keep returning to these two. Incidentally when preparing for scaling a production gin I was worried about supply issues. I happened to be visiting Magic Hat Brewery in Vermont and asked John (Brewer, Head of Ops) is they had ever had any supply issues with either of these vendors. He said he hadn't and they often place very large qty orders multiple times a year.