Hi guys,
Where would I find a replacement charger for a https://www.anker.com/ca/products/variant/Astro-Pro-20000mAh-Portable-Charger/79AN7906-BA
A quick google search only found chargers for the 15000mAh and I don’t know if it’s safe to use that one.
It looks like you can find some of this rare item listed on ebay!
If you have the old charger it will tell you what voltage and amperage is the output. You can then find a charger with the same specs. The hard part will be to make sure the jack matches so it plugs in correctly to the Astro.
There are some great deals / offers for newer chargers. 
But if cost is a factor then you may have to resort to the likes of eBay etc.
There are hundreds of generic/universal AC adapters available on Amazon or other stores. You need something that has 24W capacity or higher, since your Astro charges with 12V at 2A. Something like this will do the trick: https://www.amazon.com/SoulBay-Universal-Interchangeable-Traveling-Electronics/dp/B074BYLT19/
You just need to use the correct size tip and make sure the polarity is correct (all electronics that charge by AC adapter will have a polarity diagram somewhere on them)
Thank you very much @tonicboy for your link. That charger even has the ability to switch from US head to my required AU. Before I lost my charger that came with it, I was also using a US to AU adapter which was making it quite bulky.