From my reading of recipes, Boiron appears to be the most widely used brand, but in the U.S. it seems to be somewhat difficult to find, at least for someone buying retail. A Google search reveals widespread availability, but when one checks more carefully, often an online store will have only a few Boiron flavors actually in stock. I've yet to find one that carries anything like the full complement of Boiron products.
I have used Ravifruit (also from France) and found it very good. Sicoly is another widely available brand--it appears to be the primary one with which some companies have replaced Boiron. Since Boiron doesn't have a significantly higher price, I am puzzled as to why suppliers are switching.
The whole question of shipping and quality is another matter, and as a small purchaser of the products, I can only trust overnight delivery to do the best job possible. I try not to think of how the purée got from France to the U.S., then from the seller's warehouse to a U.S. airport, then from Dulles Airport to my little town by truck.
You could, of course, always make your own, though you might have some slight difficulty getting a good supply of passion fruit or yuzu!