A question I can finally answer. I'm addressing the "where" as there is one main place this can be done which is
upwork.com. You can find most people there as its the largest freelancer site on the planet.
Second would be this very forum as we are all interested, though not all available for work, in TLA+ while all at different levels of usefulness with our TLA+ skills (or lack thereof). However, there may be rules of the group that prohibit such actions. Maybe, maybe not.
Additionally, LinkedIn.com may produce some results, though I would image it not as fruitful as TLA+ is not common from what I can see.
I would venture to say these answers are obvious but I'm surprised when people say they've never heard of
upwork.com so I will not assume and just give the information asked for.
Yay! I may have been helpful the first time in 4-5 years of being in this group :D