I don't know anything about longarms but others will be able to help you.
I am picky when buying fabric. I like name brands like Moda, Michael Miller, RJR, Kaufman, etc. They have never disappointed me. My favorite place (or one of them) is Missouri Star Quilt Company. They sell quality fabric and shipping is low. I also like Shabby Fabrics, and there are more but those 2 jump out first.