I picked some off Ebay for less than $10 each to use in the minis that I have built. For me, they are only a backup. Given the small size of the mini WSM, unless you mount it on the lid or set the entire smoker up on top of something, the thermometer is so low to the ground that reading it is very difficult. I installed the little eyelets from BBQ Guru so I can use the Maverick probes. Something else to consider is the length of the stem as well as the placement of the thermometer itself because the longer it is the more difficult it will be to get grate(s) in and out around it.
WSM, Performer, 22.5 & 18.5 OTG, SJG, mini WSM, Summit 420, Q100, C-G Akorn, and Schwenkgrill