The three "official" places to buy them are all good - Nike shop, Seahawks.com, NFLShop.com.
The only other place I buy stuff is Fanatics. They have a very very good selection, better than the three above for most items, and a very good shipping policy. And if you buy through this link
a percentage of your purchase goes to the charity set up by GM John Schneider, thanks to "Hawkblogger" Brian Nemhauser.
I have bought items from all four places I mention here but I would try Fanatics first
because of the selection and the charity thing, and if that fails, then Seahawks.com, NFLShop, and Nike in that order.