A kink relationship is still a relationship! The anon’s Master is still their partner. Unless the Master has been specific, gifts don’t have to be kink related. That doesn’t mean you can get them socks or a tie! But otherwise think about what you’d get a well-liked vanilla partner and consider getting him non-kinky that you think he’d enjoy receiving.
As for the fellow Slave, If there’s not a good leather shop in your town and if you’re comfortable shopping online, and especially if the other Slave is a man, Doghouse Leathers in Seattle has very good, ethically-sourced, fairly priced, and often locally-made bondage gear. They also ship internationally. Note: I’m not at all affiliated with them but since I used to do leatherwork for a living I visited their shop this summer. They do good work and they’re nice people too.