There are a number of public markets in the colonies, meaning you can get supplies easily. Of course, paying for goods is another matter as currency was incredibly complex. People paid in English, Spanish, French, and Portuguese coins, wampum, New Jersey and New York bills, magic beans, Laundromat coins... the list goes on. We've simplified everything down for you in the Animus. (You're welcome.)
While some markets sold a variety of goods - like Faneuil Hall in Boston - others were more specialized. New York's Coenties Slip was mainly a fish market, (though like most markets it sold a little of everything), while if you were in the market for African slaves, Peck's Slip was the place to go.
Yes, slavery was legal at the time, though it was less common in Boston than New York, which had the largest slave population in the north. Don't be surprised to see European servants for sale, either - some were sold for a fixed number of years to pay back their passage to the New World - a much better deal than the slaves were offered.