Coming from someone who had so much fabric that I needed to give a lot of it away to different organizations and women at my church. I worked in fabric stores for years. Even now I feel like I have a lot of fabric still that I may never get to sew through all of it. I would buy what I like with no project in mind. Doing this you end up with a lot of fabrics that are not being used and just stored. I am finding now that I want to upgrade my fabric stash to more modern prints so what to do with my current collection? I started making pillowcases (to donate) and scrappy quilts to use up these older fabrics.
I would say that how you build your stash will depend on what type of quilter you are. Either you will buy for projects or buy by collections or buy what you like. Think about what you want your quilting journey to be and plan your stash building well so that you do not end up with a bunch of unused, hoarded fabrics.
There are lots of online places to buy fabric on sale if you cannot find it at shops near you. I buy most of my fabric on sale or at significantly discounted prices. I have also scored some treasures from estate sales or destashing sales of other quilters that I find on Craigslist. Craftsy often has some great clearance sales on fabric bundles. Try some of the different stash building methods mentioned to see what works best for you. This may be the best way to find what will work for you.