Local chocolatier Taza, Somerville based, sells bulk chocolate, very expensive. I'm not a huge fan of the texture of their bars, but that's just me. No question it is high quality. If you like a certain brand, search for their website and order in bulk if they have it available. (e.g. you can buy 35 lbs of Ghirardelli from the website for around $7.20 a lb; Valhrona has a wide selection for higher than that, some in bar form, some not; Lindt sells not quite 1/2 lb bars for a little over $5).

Otherwise here's a Serious Eats article about good chocolate bars, many of which I have seen at Whole Foods or other higher-end stores like Pemberton Farms or Cardullo's in Cambridge.

https://www.seriouseats.com/baking-with-chocolate