You will find it in most well-stocked Asian markets in the Japanese section. You can purchase either teabags or a granulated freeze-dried powder that comes in a little bottle that looks a lot like bottled yeast. My last bottle came from the tiny Apsara market in Herndon. 291 Sunset Park Dr, Herndon, VA 20170, in a little industrial park off of Spring Street between Herndon Parkway and Fairfax County Parkway.
I buy my miso-with-dashi at Grandmart in Sterling; I go through a lot of it as my hot liquid of choice at work when I'm feeling under the weather.

I'm pretty sure I've seen both bonito flakes and kombu at WFM. That's all you need for dashi other than water. You can make just a very little amount in around ten minutes once you assemble ingredients, strainer, jar, pot, wooden spoon and lid. It keeps for a couple of days; this recipe says three.

I recently picked up bonito flakes at Dean & Deluca in Georgetown.

!!! Are they gilded??

That's where I've gotten them.

I happened to be there for something else, and there they were. Not overpriced either.