My first choice would be the one on 12 de Julio immediately opposite the Post Office. Has honest prices although not a huge selection. Now run by the widow of the gentleman who used to run it, who died not so long ago. She is helped by her brother-in-law. If they don't have what you want it might be an idea to strike up a conversation & ask if they could get it. Most shops here will do that sort of thing if you take time to enquire about the family etc.
Second choice would be Soler on Beller - much wider selection, prices not quite as competitive IMHO, but very helpful people. If you're looking for silver, larimar etc then Fine Gift Centre on 12 de Julio but tourists go there so bargain.