Reddit mentions of MEAN WELL LRS-200-5 200W 5V 40 Amp Single Output Switchable Power Supply

Since I don't know what items other than the 15 Arduino you need to power (such as LEDs or motors or ...), I'll give you some options for 5V DC route.

When shopping for a power supply, you should try to buy a one that can supply a minimum of 1.5 to 2 times more current than you need. First it's a good thing to have extra, just in case you need more, Second you may need extra overhead at power up because of short current surges when everything first starts up, Third you don't know how well any specific power supply model might be able to regulate at maximum current rating. I prefer 2x minimum multiplier when I buy power supplies for myself.

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Amazon sells lower current models (5A and 10A) but are close to price of 18Amp model.

$19.95 - 5VDC 18A :: https://www.amazon.com/dp/B018TEAPRQ/

$28.50 - 5VDC 40A :: https://www.amazon.com/dp/B0131V99BA/ (most likely overkill)

$30.24 - 5VDC 8A, 12VDC 3.5A, -12VDC 0.5A :: https://www.amazon.com/dp/B01MYS5133

Also, you can find power supplies with 2/3/4 different DC output voltages too.

Information about each model - http://www.meanwellusa.com/productSeries.aspx#

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The above power supplies don't come with an AC wall power cord, so either cut-up a power cord you have a school, or purchase one. You can either strip the ends and use bare wire, or add spade terminals like the following.

https://www.amazon.com/Parts-Express-Power-Spade-Black/dp/B077XRW1YG/

You can buy spade terminals at any automotive store, big box lumber store, walmart. If you don't own a crimp tool, and have a Harbor Freight in your city, then you might want to consider this tool. Sign up for coupon by email and get 20% discount coupons from Harbor Freight, also it's possible the following might go on sale at any point too.

https://www.harborfreight.com/ratcheting-crimping-tool-63708.html

If you need spades, then you might want to buy a kit with many styles, either from Amazon or locally.

https://www.harborfreight.com/150-piece-terminal-and-connector-set-67683.html

https://www.harborfreight.com/1012-piece-terminal-assortment-67687.html

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If you don't own a multimeter, then you better buy one, because you need it to test your the wiring after you assemble everything.

Hook up things in incremental steps and validate to prevent destroying multiple arduinos at the same time.

1. first hook up AC wire to power supply, then test output voltages of power supply.
   
   
2. next hook only ONE USB cable, then test output voltages at USB connector. Compare against similar cable plugged into a computer to make sure location of 5V and Ground are a correct match!!!!
   
   
3. next hook up to only ONE Arduino that you can sacrifice to verify the power is correct, make sure it powers up properly, then text voltages on Arudino board.
   
   
4. next make all USB cables, test one at a time with the same ONE arduino.
   
   
5. next hook up all 15 USB cables to power supply, next probe the ends of all cables to make sure location of 5V is correct, then use the ONE arduino to test each cable.
   
   
6. next go through all 15 arduino, and test ONE arduino at a time on any one USB cable.
   
   
7. next try hooking up a subset of Ardinos and test system, maybe try 5, then 10, then 15, or what ever feels right to you. After you hook up each group, leave power running for a while, maybe an hour, then check voltages at the power supply output and at all boards. After you hook up all 15, "burn in" for hours then check everything again.
   
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   You might want to hook up a dedicated voltmeter to the output of the power supply too. Use this so you can easily see the voltage while it is running in the display. During assembly above, use a multimeter.
   
   $10.80 :: https://www.amazon.com/dp/B073GG3DWF/ (kind of expensive from Amazon)
   
   $1.52 :: https://www.ebay.com/itm/171514345083 (China, 3 weeks to 2 months to receive)
   
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   If you need various electronic parts, such as resistors, capacitors, LEDs, for a cheaper price than Amazon, but you don't want to wait for a long ship time, consider Tayda. They are very popular on Reddit, do a search on Reddit. It usually takes about 6 to 9 days) to receive parts from Tayda to USA, depending on what day of the week you order. You can get a 15% discount code from them on Facebook about once a month. https://www.taydaelectronics.com/
   
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Thank you for your reply and help. Really appreciate it!

The original power supply was specified to be (8) of these:

Brand:Meanwall
Power:200 W
Voltage:5V
Use voltage:220 V
Current:40A

https://www.amazon.com/MEAN-WELL-LRS-200-5-Single-Switchable/dp/B0131V99BA

However, these require AC input and therefore are not portable from my understanding. That is why a battery is preferred.