Torani Amer is a good sub for Picon if you live in the US. The makers claim it was directly inspired by the French aperitif. I've never tried Amer Picon, but I do know a Brooklyn made with Torani is pretty great.
Agreed. As an update a few years ago Torani decided to exit production of this product and gave the rights/recipe of their Amer liqueur to an Italian family company; it's now made by the Ferino Distillery in Reno NV, and called Ferino Amer.