As promised before last year’s Burn, I’ve finally put up the build log for the new version of the Technocolour Dreamcoat. Lots of pictures! Lots of words! Hopefully contains some inspiration and maybe even practical advice.
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Thank you!!! About to embark on a similar quest for a project, and the power stuff is always such a sticking point for me.
I would love to get 5-6 amps from a pack and run two packs with two power rails, but finding that outside of RC applications is neigh impossible 
Yeah.
But, consider: USB-PD trigger cables exist and you can get 3A @ 5V from one (I’m not recommending that particular item, I was just searching for an existence proof) and for that you get nice safe UL/CE-certified battery packs from Anker that are easy to charge. 6A @ 5V is a lot of power and your LEDs will be obnoxiously bright. There’s a nonlinear relationship between power draw and perceived brightness; you’ll hit diminishing returns from about 80% of the rated power draw of the LEDs and will have almost no chance of setting yourself alight with alkali metals not even a professional firefighter can extinguish.
As I said, I’m attempting this as a stunt (… so nobody else needs to.)
So… yes and no. I have found that when using faux fur, like really thick fur, you need the brightness because of the diffusion levels. I would like to run 1k+ LEDs with at least 50% brightness, but maybe then I’m just asking for a fire to start.
You might need high brightness, but do you need high pattern density? Can you have most of the pixels off at any one time?
Right; also consider the colour saturation of your patterns—full red is only 33% of the rated power and full purple only 66%. It’s only full white that gets you to 100% of your power draw and you rarely want to do that in practice.
For a bunch of reasons I’d advocate a “progressive enhancement” approach: do it with plain 2 × 5V @ 2A USB-A power banks first. If that’s not bright enough upgrade to USB-C PD trigger cables for 2 × 5V @ 3A (a third more!) and if that still doesn’t satisfy only then go silly.
(And maybe look into LiFePo4 battery chemistry, much less explodey, but harder for me to get hold of where I am.)
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Yeah, I am looking at starting with 2 usb c at 3 amps, which should be enough most of the time, but I like to provision for worst case. Definitely have patterns in mind that will be way under the max draw.
Will get things going and then in a few months see if I can post some helpful info for others who want to do this 
I love this project in part because of how overboard you’ve taken the power requirements – It’s the exact opposite of the approach I took with my coat – Beginners Guide to Making a LED Festival Coat – which was / is built on just a VERY small battery and staying under 1a at 5v at all times. @hex337 – if you take what @ratkins did as one level of “this is hard to power” and what I did at the “this is the easiest way to power” you’ll probably land somewhere that’s the right complexity level in the end.
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Separate from my prior reply about my own coat – these exist at the standard very large markup from Adafruit, if you want something reliable, shipped from within the U.S., and with better documentation than Aliexpress.