You willl have to find someone interested in your cause, ( assume the same reason you and others don't get paid for your work)
*** now I am assuming you don't have any paid people in your group or non profit ?
If your janitor or your manager gets paid, why do they accept money and not work for free ?
Basicly you will advertise that you are looking for someone to help your non profit on a volunteer basis.
But remember you most likely will not operate as well without grants, so a good grant writer is worth their weight in gold, since they are the pepole who keep many non profits operating