Where do you buy chorizo in Boston?
Formaggio Kitchen (Huron Village), Formaggio South End (South End), and Las Ventas (SOWA), consistently carry Spanish chorizo (dried chorizo), in both mild (authentic) and hot variants. Price is around $9 for a standard-sized link. I don't know if that price is competitive, but the quality is.
Via Formaggio Kitchen, you can also order them online.
244 Huron Ave, Cambridge, MA 02138
While my Portuguese heritage has me buying chourico and linguica often, I agree with galka that Spanish chorizo is very difficult to find in Boston. I have found it once or twice at Whole Foods, forget the brand, but it's hit or miss. My wife was buying some from the meat vendor at the Belmont farmers market this year. That is very good, and is $8 for 3 links. They have it on Thursdays, I think through October.