At the airports a lot of Hudson News stores will have them. Around town you can often find them at check cashing joints, independent pharmacies, some bodegas, etc.

Here's the the MTA's retail locator. Note, however, that there's no guarantee any particular store will have the 7-day pass in stock. Unlike Metrocard vending machines, which can load a blank card with any amount of $ or time, the retailers sell preloaded cards in set denominations.