I’m looking to fit a dropper post to my Sonder gravel bike. Is there a specific lever that I can get to work with drop bars, and can I use it with a brand X post?
TIA
If you're using 1x you can just use the left shifter to control the dropper post.
Although you might have to buy a new shifter - my Force 1x group set has a non shifting LH shifter.
I’ve got the brand x bar remote fitted near to the stem, works well for less gnar stuff but not ideal for more gnar or trying to go quick, also gets in the way of my handlebar bag when bikepacking. Have got the Crank Bros highline drop bar remote, not fitted yet but goes inside the left shifter (I’m 1x SRAM Force), means your able to be on hoods or drops and operate the dropper (so same as the Shimano dropper/brake shifter, but “only” £45 for the CB remote (Evans still have them)
My 2c 👍
My Fugio has Shimano GRX and a cable operated dropper post on it. The left lever is designed for the job.
https://www.freewheel.co.uk/shimano-grx-st-rx810-la-grx-dropper-post-lever-left-hand-strx810la

I am (literally) about to fit one of the those Easton remotes...
Also, if you're running 1x but have a 2x left shifter to you can actually butcher the internals to remove the ratchet, then use the left shifter as the dropper control.
Note: I haven't done it but, randomly, I watched a You Tube video last night showing how it's done
Have one of these on my gravel bike, works perfect
Thanks for all the suggestions