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Taobao
Taobao is a one-stop site for all things cheap. Yes, we're not even exaggerating! This site, which is originally based in China has just recently created an english based site that eases the convenience of every non-Chinese reading customer. They sell almost everything that you can ever think of, from tech devices to tidbits and snacks, you name it they have it.
Taobao has quite a handful of stores selling party supplies and they range from stores selling only balloons, to stores selling almost everything, to stores selling specific themes. They also sell air pumps and photo booth props as well!
For more information, visit their website.