Where to find critique for query letters?
Pitch wars (an annual event pairing aspiring fiction novelists with people in the industry) is imminent and this link leads to the 'middle grade query letter' message board for entrants and mentors (and anyone interested). There are other boards for young adult, adult, etc. The boards are quite actively critiquing queries for the next six days, when Pitch Wars begins. I'd suggest posting there under MG fiction--you'll get feedback whether you hope to enter Pitch wars or not.
This link leads to the Absolute Write query letter feedback board. I believe you need fifty posts before sharing your own work on Absolute Write. I am not certain. There's encouragement to give help to others toward reaching the 50-post mark. Middle grade is tough to write well, and there are people with MG fiction on the site. As a note of warning: You will never 'finish' a query on this link, the board is active enough that even a perfect query will continue to have critiques. But they also have a list of 'successful queries' that 'worked' to land agents.
There are likely other resources. Query Shark and Query Kombat are online resources, probably better feedback (professional) but any entrant is less likely to be selected (they pick a small percentage.).