Where to get materials in Boston area

[Gurnet76]

I would recommend checking out Defender. It's down in CT but the prices could make the trip worth it.
https://www.defender.com/warehouseoutletstore.jsp
They usually do a big sale in March.

On the South Shore I use New England Marine Industrial(Brant Rock) (and occasionally West Marine) for those types of products.

As far as a tank builder, it depends on what you need. For a welder to create a tank to spec I've heard others have used Kent Fabrication and Fastech Marine Fabricating.

I haven't had to cross that bridge just yet but would be interested to hear other's recommendations.