Swag codes are random and expire soon after they're released. They are hidden within the messages that Swagbucks post on their website or on their Facebook or Twitter pages. Anyone who promotes swag codes that do not expire are working outside the rules of the Swagbucks site.
The Swagbucks website lists codes throughout the day and there is no list of previous swag codes. There are apps and browser extensions available through their website to use to get notified of new codes. Some online groups announce new codes when they're available. Swag codes can be used to build up Swagbucks that can be redeemed for gift cards or Paypal cash.