I got caught up in the publishing vice and often tapped into the graduate student pool when I was looking for writers with the research chops to delve into one of my juicier subjects. One of the most effective tools I discovered was to get professors I am friends with to introduce me to professors in the field that I was looking to cover. Once I had my 'in', those professors were always extremely open to introducing me to graduate students they personally know, or more generically, introducing me to Graduate Students' Associations (every University I know of has one, or something similar).

If I didn't know any professors, I would likely go directly to Graduate Student Associations, though be prepared - higher education can feel rather closed off, so they may view you with suspicion until you get your in!

When I was in graduate school, my department's website maintained a list of students who signed up as "tutors", willing to tutor undergraduates in exchange for cash.

Maybe this is less common outside of math (as there were a ton of low-level math courses), but this was surely a reliable way of finding grad students looking for paying work.

But warning: if the projects are ongoing, I feel like you might need to pay well, at least $30/hour. My suspicion is that anyone who would accept less is liable to not be very busy (never a good sign in a grad student).