I have a Korg Orchestra Tuner that works pretty well. I paid around $90.00 dollars for it. The tuner pickups that they sell in the harp store have never worked for me. My teacher uses a guitar tuner pickup that she sticks onto the soundboard that gets the job done well.
I also use the Korg Orchestra tuner, and like it quite well. The only drawback is that the speaker is in the back, so if it is on your stand facing you, and you want to tune to its pitch, it is muffled. Korgs are always good for producing a pitch that is easy to tune to, and it meters all the strings, though you may have to move the pick-up. It doesn’t come with an adapter, which is worth paying for, as batteries only last about 3 months. I do not use a suction cup pick-up on my sounding board, it leaves a mark.
Stick it into the Input jack, clip it onto your harp (a rib works great on a L&H, but wooden ribs are often too thick, so you can clip to the edge of a sound hole or even to a tuning pin), and tune away.