Not sure how familiar you are with Lego pbot, but generally the way they do it is they have certain sets they release each year, and then the next year they release a whole new bunch of sets for each theme. The Lego store in particular will almost never have old sets (at least from what I have seen) but sometimes you can find them at toy stores, big box places like Walmart/Target, etc.
You can find a lot of the older sets on Amazon, or Ebay also - if you can find out the four digit set # it will make your search a lot easier - I go to
www.brickset.com, it has all the numbers.