There are weekly tournaments at Uncle's Games in Redmond Town Center! You can get booster packs and (when available) promo cards. Here are the details from the store:
"Come join us for casual play of the My Little Pony Collectible Card Game. Solve problems in Equestria with the goal of being the most useful pony. The first player to solve 15 points worth of problems wins the game! This is an all ages event, remember friendship is magic."
The store's description is outdated as we've been having tournaments at this location as well as Bellevue's. It's swiss format, $5 to enter, and everyone gets at least one booster pack.
Note: This meetup listing recurs monthly, however the event is weekly. Despite RSVPs being turned off, you will find a consistent group of both meetup and non-meetup players.