The general brand of mirin (sweetened sake) is Marukan. Kikkoman also makes mirin, but watch out for "Aji-Mirin" which has msg as a seasoning in it. I don't like that. It does season sushi rice properly but I don't care for msg in anything.
Most stores that carry rice vinegar have Marukan, if not, you can find a Korean grocery probably in your area (they are fairly common, even in smaller towns) and they will have Marukan and Kikkoman. You don't need to fuss over mirin the way you do over a good sake, not that I can find a good sake in this state, where they control which liquors you can buy.