I saw some Harry Potter related itens, as well this 101 Dalmatians cape [item=Giselle D.E.V.I.L.L.E Spotted Cape] How do you find itens with a specific theme? D: Is there any list or key word?

Generally, you just have to search by words that might get you what you're looking for (ie, for more Dalmatian items, "Dalmatian", "spotted", "black and white"). That cape is also part of a line, so the rest of the Giselle D.E.V.I.L.L.E items are a good place to start as well.

For fandom-specific stuff, there's a list HERE you can check out.

Thank you <3