GB - I've yet to find them in my local markets here in Culpeper, but our local markets are constantly expanding their Asian aisles, & more & more I'm able to find key ingredients. I have a feeling Hon-Dashi won't be far behind.
Any Asian market would definitely have it, & I'm also sure it's available on-line. I've tried a couple of brands, but my hands down favorite is "HON-DASHI - Bonito Fish Soup Stock", distributed by Ajinomoto U.S.A., Inc., of Portland, Oregon 97203, Product of Japan. The quote on the box is that it's "Japan's No. 1 brand seafood soup stock made from dried bonito's essence and other flavorings". Box is white, red, & navy blue. It truly is delicious & makes a fabulous base for any Japanese soup. It's actually been recommended in Japanese cookbooks.
The box contains several envelopes - enough to make LOTS of soup - so one just takes what one needs, then places the opened envelope in a plastic bag & pops it into the fridge, where it lasts pretty much indefinitely.
Definitely worth seeking out. One box will last you a long time