If it's not a major brand name, or the price is lower than Costco, it's probably not authorized. (Costco sells below wholesale cost because they make their profit on the membership fee. Anyone selling lower than Costco is almost definitely a bogus reseller and you are getting pirated software, or software with malware, or they will just take your money and run.)
Unfortunately there is no published list, but places like Amazon. Staples, Target, Walmart, Costco, OfficeMax, Best Buy. For online computer sellers, I think NewEgg and OWC are authorized.
I'll report this post to the moderators for investigation by security. Unfortunately, top google hits are paid for, and it's hard to stop them.
