BJ and roulette are two games where it's easy to end up with extra $1 chips. I personally tend to hold onto at least one $1 chip from casinos I've played at as a souvenir, they're nice to keep track of where you've played in a tangible way. My collection currently boasts chips from Vegas, Atlantic City, Norwegian Cruise Line, England (1 pound), Estonia (1 Euro) and Poland (1 Zloty).
You generally can't buy real chips from a gift shop. I've never seen it, personally. But as mentioned about holding onto leftovers, that's the way most people do it. You can technically just lay a $1 bill on the table and buy a chip from the dealer, but to me that doesn't have the same sentiment as chips part of what you were actually playing with.
So to answer your question, play as you normally would, pocket a $1 chip whenever the opportunity presents itself, and you're good!
