Where you buy hair depends on the project. Bundles for sew-ins, hair for wigs, braiding hair, and clip-ins are not the same purchase.
For sew-ins and wigs
Look at bundle retailers with clear length, texture, origin, return policy, and customer photos. Better bundles should have full ends and minimal shedding.
For synthetic hair
Beauty supply stores are convenient because you can see color and texture in person.
For custom wig making
Buy enough hair for density and length. A wig often needs bundles plus a closure or frontal.
Bottom line
Buy hair based on the finished style, not the cheapest bundle price.