I recently had someone contact me with this exact question. Unfortunately the answer is: nowhere, yet! You can’t buy a Fiat 500 Abarth in the US yet because they haven’t even started building them. The three models of the base Fiat 500 haven’t even gone on sale, so it may be awhile before anyone has a chance to get an Abarth. Hopefully sometime this year will be possible.
I’ve recently added an FAQ page to hopefully answer common questions like these. Keep asking and I’ll try to answer as quickly as possible!