Merton Fiberglass in Springfield Ma. has all the glass,resins,paints,fastners,etc.It would be a bit of a ride for you to that part of the state but he ships daily and it would be a one day delivery to you.
If you needed a fair amount of stuff the 2 hour ride may be worth it,but heck, why waste the gas,it would cost less to have it shipped
www.mertons.com
I used to be able to get marine plywood from North Pacific plywood in Spfld as well,but they closed.
Any good high quality independant lumber yard should be able to order the plywood for you.
***OOPS!! somehow I forgot that you said Newburyport area,for incidentals and stuff you forgot. Sorry for that. My apologies.**