Good advice from a lot of people. Especially since you’re a non-profit charity, try to find people with good character/reputation that revolves around the industry. Some “influencers” are awful people and just want to make a buck, but I’ve developed relationships with some who just post for us because they like us.
Personally, I don’t know if there is a lot of ROI if you’re dropping cash. Find hashtags for the demographic you’re reaching out to and see who is local to you. Any “influencer” from outside of your immediate geography is likely not worth spending any money on.
Word of mouth is definitely key. Best of luck!