The services exist, google "critique service" (without the quotes). Ignore the "copyedit" services (if that is all they do), they are just checking spelling and grammar and formatting.
Actual critique services are just damn expensive; running roughly $100/hr, or if the work is long enough, around 2-4c per word, OR $10.00/page for short works, or $8.00 per page for long works. I know you are writing essays, but if somebody wrote a 90,000 word novel (typical) that would be 360 pages, and by these prices anywhere from $1800 to $2900 for a critique.
They are easy to find, most will give you a quote on a word count.
An essay of 5000 words or so may not be bad, but still perhaps $200.