Home depot or lowes may carry something for a pinch may even have stainless. In Taunton area you can try Manny's hardware that freaking place has every thing. When ever we need a special bolt or nut for our rubbish trucks some stuff is huge no one carry's it (can get through Fastenal but they only sell in 50-100 packs) we run over there and laugh when you walk in guy comes right over to help you (and unlike home depot the are very knowledgeable) hand him some huge grade 8 bolt he looks at it for second and says oh yeah right over here walks to the exact spot in one the isle where it is pulls it out 8" long bolt with inch and half head on it. I don't ever remember going there and not coming back with the right stuff or they help you find something that will work. There is a reason they are one they few private old school hardware stores surviving in the home depot age.
rant over.