I have had good luck with Rick & Ann at Ship Shape Marine works in Gilford.
Ann has this lifetime thread she used to re-sew canvas work I had done at Village Canvas that failed after just 2 years, where the zippers attach.
She also did a great job on a complete camper pkg. on my wife’s Chaparral.
And my wife is VERY picky about here baby.
Lots of places out there, good luck.