I can’t tell you in this email where (as in what mill or supplier) to source fabrics from because it depends on your product, price point, preferences, and so many other factors.
What I can tell you is where (geographically) to source fabrics from.
You want to source fabrics as close geographically to your factory as possible. This cuts down on shipping costs, production timeline, and environmental impact. If you are producing domestically in the US, ideally you can source fabrics that are made (or at least imported and warehoused) in North America. If you are producing abroad in another country, try to source fabrics from that same country.
What you are trying to avoid is, for example, buying fabric made in Asia from a company that stocks the fabrics in the US, only to ship the fabric to your factory in Europe, who then ships the finished garments back to you in the US. There is a lot of wasted travel time and expense in this scenario.
Wherever possible, avoid shipping materials back and forth through multiple countries during production. The fewer trips the individual materials and finished garments need to make, the better.