I have had good luck buying RK Kona solids from
https://www.fabric.com/
They have good everyday prices ($5.50-6 with free shipping with $35 order), but also have sales sometimes where the fabric is even cheaper. I get much better quality Kona from them than what's sold at JAF (dye isn't consistent from bolt to bolt, fabric is wound really crooked & I lose over 1/4yd squaring it up, snags, etc). I usually can match what I've bought at LQS with yardage from Fabric.com with both Kona & Bella solids. You might check them out & see if they run any kind of special this weekend.
Good luck! Other than that, I might see if you could get donations of fabric. Around here sometimes people who don't sew/quilt will donate money/fabric to help contribute. Quilt shops & big box stores (JAF, HL, Hancock's) will sometimes donate to 501(c)3's with a formal written request from the charity. Otherwise, I wouldn't really feel badly about using JAF fabric for charity quilts. It is lovely if you can afford to use quality fabric, but even big box fabric has it's place in the quilting world. I've made quilts for my own family with JAF/HL fabric (before I "knew better") and while I now prefer LQS fabrics for a number of reasons, those quilts have held up well for 2 years & counting with regular washing. They are not heirloom quality quilts, but they are loved, enjoyed & beautiful in their own right.