I would like to buy some Esperanto stickers for my laptop and maybe even some t-shirts and a mug. Is there a place to buy this sort of thing?
I'm a big fan of La Sekreta Societo de Esperantistoj. They specialize in T-shirts and stickers. I have their stickers on my laptop and my car.
Look up CafePress and go to their website. When you get to the main page, search for Esperanto. There are quite a few shirts, mugs, stickers, and such sold there.
Your national Esperanto organisation is probably the best place to look.
I have seen such merchandise being sold at bookshops at international Esperanto events, e.g. at IJK and UK.