Though you are accustomed to using quality cotton threads for patchwork, clothing construction is different.  It's still fine to use cottons for any patchwork in your garment pieces, but to actually construct garments, you need a good quality polyester thread.  You can find less expensive store-brand threads, but my experience is that those are fragile and do not survive use.  Use a 50-wt. thread in a color that matches or blends with your fabric.