) has a lot of cute fabric. . .I haven't actually *bought* anything there yet, but I love to browse there for inspiration.
Nearly all of my stuff so far has come from my local fabric store (Joann's) or a thrift store.
I agree with sweetiepickles; often the quality of the fabric can make a big difference. I like to feel the fabric, check how it drapes, see how heavy/tightly woven it is, etc. to get a feel for how it's going to hold up and how it's going to look in the end. Also, as I learn more about the qualities of specific fabrics--twill, poplin, interlock, etc.--I find it gets easier to locate the appropriate type of material for the project I've got in mind. In general I've found that quilter's cotton is fine for light/casual clothes, but heavier apparel fabrics will usually look a bit more "dressy".