And it's widely available, so you won't have to scour to find it... though the ubiquity means some brands are better than others.
"My favourite is the Higashimaru brand," says chef Hana Etsuko Dethlefsen, culinary instructor and author of Let's Cooking: Japanese Cooking at Home, Vol 1. "If you can't find that, the Mizkan brand is usually easier to find."
There's also this incredibly addicting looking Yuzu It sauce, which is "essentially a yuzu-chili hot sauce. It is more versatile, mild and user-friendly than traditional yuzu kosho," says Hart Lowry, executive chef at Hojoko in Boston.
If you're looking for a more refined product, you can always make your own, too. Chef Hiroyuki Naruke (aka Chef Hiro) at Q Sushi in Los Angeles recommends that you buy "any brand that is a basic blend, and blend it with kombu seaweed and bonito flake. Let it wait a few days to marinate. This will make your ponzu taste better by adding in the necessary flavors."