Hi Ace,
I am Sumit, an Independent Advisor and a 2-Year Windows Insider MVP here to help.
As 1903 spent more time in testing, I believe it is as stable as 1809. However if you want to install 1809, you can find the ISO here:
https://tb.rg-adguard.net/public.php
The downloads are fetched from Microsoft servers so the authenticity is not an issue.
Disclaimer:
This is a non-Microsoft website which would provide accurate and safe information. Watch out for ads on the site which are frequently classified as a PUP (Potentially Unwanted Products). There is no need to buy paid products to fix your computers as they do more harm than good sometimes.