Lots of them on amazon. Not 100% sure it is a 5521 connector but I agree, it is most likely. This one would be the correct power supply: https://www.amazon.com/100-240V-Transformers-Switching-Applications-Connectors/dp/B077PW5JC3
Do you have access to the inside of your case, and the standard 4-pin Molex connections instead? 2 of those wires are ground, one is +5v, one is +12v.
If you want to go to the wall, then as someone else already pointed out start looking at old wall-warts and find one that's 12V, maybe you'll need a coaxial pin adapter if you don't want to cut the wires on the adapter. You can find this sort of thing cheap as heck on Amazon too, just need to find the right size. e.g. https://www.amazon.com/ZRAMO-Terminal-Adapter-Connector-Security/dp/B06XXGH1HZ/ref=sr_1_18?dchild=1&keywords=12v+coaxial+terminal&qid=1594230456&sr=8-18
Example of a 12V wall wart with the 2-wire adapter: https://www.amazon.com/100-240V-Transformers-Switching-Applications-Connectors/dp/B077PW5JC3/ref=sr_1_3?crid=2W5P10RGEWKHO&dchild=1&keywords=12v+wall+wart&qid=1594230606&sprefix=12v+wall+%2Caps%2C193&sr=8-3
p.s. neither link is a "buy from them!" recommendation - I didn't research the seller more just did a basic amazon search and linked a likely quick example.
I actually just did this a couple days ago. Here's the items I purchased and links to their page on Amazon. You can also purchase all of these items from Konnected's website.
I attached the female DC power screw terminal adapters to the end of my alarm keypad wiring in the alarm box. From there, plugged in the 12V 2A AC adapter into the wall and connected that to to the Y adapter which then plugged into the DC power screw terminal adapters. The AC adapter I ordered on Amazon came with two so I just have the other as a back up in the event the one I'm using ever fails.
At the keypad locations, I then wired up the 12V to 5V USB power convertor using these solder seal wire connectors and a heat gun. You could obviously just solder it as well and use some heat shrink wraps, but I didn't want to buy solder equipment so I went with these solder seal wire connectors which actually worked out pretty well. Of note, you have to use a connector that fits the thicker gauge wire on the 12V to 5V USB power convertor so you won't get a water tight seal on the keypad wiring since it's a thinner gauge. It wasn't a factor for me though since these wires are just sitting behind the dry wall in a dry location.
I hooked these up to two Amazon Fire Tablet HD 8s (actually only have one hooked up right now) and it works well. They do sell Y adapters that split off into three ends and you should be able to charge three tablets with that as long as you make sure the power adapter will support however much power the three tablets will draw while charging.
I also ordered an Aeotec Smart Switch 7 which I plan on plugging the AC Adapter into so I can setup an automation that shuts off the power occasionally so the tablets aren't sitting at 100% all the time. Still haven't decided if I wanted to bother with this since the Amazon tablets are so cheap and I'm not sure if it's really a factor for newer tablets.