I do a lot of quilts for charity and here are my best tips.
I let all my sewing buddies know that I am doing charity quilts and they give me their scraps.
Our local Goodwill sells fabrics and I haunt the place.
Fabric stores in my area often affiliate themselves with a charitable group and give discounts for people who sew to benefit that group.
Family and friends give me clothes that they can't donate (perhaps there is a hole that can't be patched) and I use those in addition to fabrics I find along the way.
Good luck.