I would not make charity quilts for that guild.
It's one thing to ask that quality be good 100% cotton however telling folks where to buy is not their place to tell you so.
I'd recommend you buy where you want, make the quilts,and donate them yourself if they won't accept yours.
I belong currently to two guilds and one of them we have been told recently to patronize a LQS and a member of the guild who has a studio and sells AccuQuilt machines and dies. We pretty much told her to KOAS and that we would purchase our fabrics and AccuQuilt machines where we wanted to.
Personally with both guilds I'll do certain things for them but handing them quilts is not one of them to donate to whoever. I help them in other ways.
Personally I make charity quilts but I chose the fabrics I want to use and I make contact myself with organizations I want to donate them to.
No guild has any right to dictate where we buy our fabrics as long as it is good quality 100% cotton. And I can understand that part of it. Personally I feel we should make charity quilts of the quality we ourselves would like to receive.
I just finished cutting 1200 charm squares for one of the guilds and I have to say a bit of the fabric donated was awful to the point of being able to see light through the fabric. To me if I can put my hand behind the fabric and see it is not good qualty fabric. Fortunately only about 1% of it was like that. I still cut it and will let them decide if they want to use it or not.