Malee Book Centre is an underrated treasure trove of party equipment - especially Disney themed stuff. It's a small unassuming little store by the Kollupitiya junction - some of it is imported, everything is very affordable, and they've also got a good range of stationery and art supplies.
Kids will love this store. The shop is a perfect choice if you're planning a children's party: they have birthday napkins, birthday plates and hats, birthday balloons - and they even make personalized banners, and pinatas full of chocolates - themed on Disney and Universal favourites. There were a lot of minions (from Despicable Me) on our last visit, a few Monsters Inc. notebooks, a Brave table napkin, and even Frozen themed party stuff.
I found Elsa from Frozen and I bought her. I know I'm not 9, but I just couldn't... let it go (cue laugh track/ cringe of discomfort.) For the old-school cartoon fans, there's also some party equipment with classic Disney princess themes (Cinderelle & co) and even a bit of Batman and Spiderman.
Party plates, napkins etc. are in the Rs. 100 - 900 price range, depending on how many you get in a set. Stationery-wise, they've got a huge collection of papers of all textures, if you're into craft, charcoal pencils for artists (not a lot of paintbrushes though), acrylic paints, and also those Pilot pens that allow you to write in ink and still erase it (Rs. 540).
There are also a bunch of other interesting finds here and there, like flamboyant masquerade masks, Spongebob stickers, LED ice cubes that change colours while in your glass, and hair spray that can temporarily colour your hair. Also - attention, teenage fangirls - they have a small set of One Direction themed notebooks and pens.
Malee Book Centre is a great place to stop by if you're planning a kid's party. And even if you're not, if you're just a big fan of cartoons and stuff with cartoons on them - it's worth dropping by and browsing through.