How nice of you! The quilting part of the charity quilts is always the area where we need help. In my guild we can provide the batting and the backing. We hope that the quilt top maker will donate either or both, but since it normally doesn't happen we are happy to provide it.
Most of our longarm quilters only quilt a few each year, so for these we give batting cut for those quilts. We have one lady, bless her heart, who quilts a lot for us. We give her a roll of batting and she keeps it separate, using it only for the guild charity quilts.
I would suggest you contact them, explain that you are a new longarm quilter and would be willing to quilt some charity quilts if they provide the batting and backing. I'd also suggest you tell them if you are trying to start a business and ask if you could get recognition for your contributions. At our guild meeting Show and Tell we show the charity quilts and say who did the top, who quilted it, and who did the binding.
I hope this helps. And, Good Luck!!!!