I don't know about these days, but back in the 1980s there were a number of electronics surplus stores located on Canal St, below SoHo. I used to pop in now and then to buy stuff. They were part of a larger ecosystem of industrial supply stores left over from the days when SoHo was a factory district. Sadly, the last time I was down there no electronics stores remained.
There used to be some sort of frequent "Expo" - which was more of a flea market to me - that took place in Secaucus, NJ...and i bought plenty of components and a PC computer or two there back in the early 2000s. Nowadays, i just assume there isn't anything like that and just look for stuff on ebay, craigslist, used/refurbished sections of newegg, amazon, etc.
Best one I know of that might still be operating is Hall of Science Amateur Radio Club[0]...