Twenty people in the United States asked this question:

Where to Buy Calamansi?

Farmers markets and… Amazon!

MANSI premium calamansi juice — not from concentrate.
https://www.amazon.com/MANSI-Premium-Calamansi-Juice-11-2/dp/B01EMLS4R8/

Amazon also has plants that produce calamansi-looking fruits.
http://www.amazon.com/Everbearing-Persian-Lime-Tree-Bearing/dp/B007LOHG3M/

Unfortunately, in the United States, the skin or peel of real calamansi fruits turn very orange, likely due to the climate… For Philippine-raised Filipinos, it’s really weird to see because calamansi’s rind is supposed to be green!

And whatever you do, do NOT buy the Manila Gold Frozen Calamansi brand, which is a product of Ramar Foods, the company that stole the Magnolia brand and logo from the Philippines.

Long live the Philippines!