I've mentioned the Bunny Brand Hori Hori before as one of the best tools in my garden shed and since then I have been asked where to get one.
Try your local nursery first. In Missoula, I know that Caras sells them because my sister picked one up there when she visited. Online, you can get them from The Garden Shop in Washington state or if you are interested in a wide selection of Japanese garden tools or the Hori Hori in different sizes and metals, check out Hida Tools. Oh and search tip: I call it a knife, but Hida refers to it as a trowel. (You can also click through my Tip store to quickly get one through Amazon.)
Also, I want to give a shout out to the amazing wonder woman Jen Pledge Master Gardener for turning me onto the handy thing many years ago. She is a great go-to gal for gardening (and other fun things) in Missoula.
I really USE tools and my carbon steel Hori Hori has been indestructible so far. I use it for weeding the toughest buggers Montana has offered me without failure as well as the most delicate plantings like the thyme plugs.