Reddit mentions of Camelbak Products 2016 Classic Hydration Pack, Black, 70-Ounce

Walmart has some in the camping/outdoors section, as I'm sure any sports authority or sports stores would have them as well. This is mine which has one pocket for the bladder (which I put the rest of my stuff in there in a gallon ziploc bag) and a tiny pocket useful for like keys or gum or unimportant stuff. Camelbaks are allowed and don't have to follow the one pocket rule, but they can't be backpacks like this.

i always take something similar to this https://www.amazon.com/gp/product/B00G46CY08/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 and they always let me in

I got this basic camelbak, holds up really nicely after 2 years of raving and you could just find cool ways to glitter it up haha

If you're just looking for water + small storage(like 1 pocket). Look into a camelbak.

Something like this: http://www.amazon.com/Camelbak-Products-Classic-Hydration-70-Ounce/dp/B00G46CY08/

It's even small enough to wear under your jacket. Stays close to your body so you don't feel like anything is really sticking out and throwing you off balance.

I've been using the classic 2016 hydration pack for 4 years now. https://www.amazon.com/dp/B00G46CY08/ref=twister_B00J08KDTQ?_encoding=UTF8&psc=1.
Not the biggest but light enough where you can jump and dance with a full pack of water. I suggest buying extra mouth pieces as well, just in case. (https://www.amazon.com/dp/B00G46CY08/ref=twister_B00J08KDTQ?_encoding=UTF8&psc=1)
Last year my mouth piece randomly fell off when I was dancing.

https://www.amazon.com/Camelbak-Products-Classic-Hydration-70-Ounce/dp/B00G46CY08/ref=sr_1_4?s=sporting-goods&ie=UTF8&qid=1517686112&sr=1-4&keywords=camelbak&refinements=p_n_feature_four_browse-bin%3A2325765011

is what I use. Fits under my shell just fine.