Must be Stealth type
For a 1x set up if that matters
30.9mm if that matters
Usual answer for a budget option is a Brand X from CRC. Although One Up seem to be getting some love too - the V2 version where the cable actuated the dropper rather than the V1 where the outer does the actuation.
V2 uses the outter cable too unless you get the V2.1 actuator kit and that then swaps it to using the internal.
OneUp with the 2.1 actuator.
OneUp have said future posts with come with this as standard.
If you order from Leisure Lakes Bikes you can spec with the 2.1 actuator at no extra cost. Thay have the 30.9 in stock in 150/180/210. Just order the Santa Cruz version and you get the actuator in the box and its a 5min job to change.
I'd get Brand X unless I really needed the low collar height on a OneUP.