I'm wanting to get a hori hori knife. What details should I look for, what brands are good? I know I want a carbon steel blade.

I have one of the ones that was promoted here a year or so ago - https://permies.com/t/52154/Hori-Hori-Garden-Knife-Permies

It's stainless, not carbon steel.  But I absolutely love mine.  Really sturdy, exceedingly useful, and I keep it on a belt hanging by the door so I can grab it any time I go out to the garden. 

It's my go to tool for all small jobs or for carrying around with me 'just in case'.

That was a good one, Burra.  Full tang is what you want if you will do a lot of digging.

I was given this same one for Christmas year before last and I absolutely adore it. By far the most used tool in my garden. It still looks amazing, too, and is still sharp, after all this time and use !!