The best approach would be to contact the corresponding author listed on the published paper.

You can find from gethub or follow the instruction of Max Balter if you hold any authentic paper/thesis/report i.e., contact the corresponding author listed on the published paper.

https://www.quora.com/How-do-I-ask-the-authors-of-a-scientific-research-paper-to-share-the-data-and-source-code-they-used

I would suggest to request the authors to provide their codes. Having said that, it's happened may times that such request are not always answered. Sometimes, the authors happily send all the resources (source codes, datasets, parameters, etc.). There are many cases when the authors simply ignores such requests for various reasons. You could have a look at this post for details:

https://academia.stackexchange.com/questions/26159/can-i-request-the-code-behind-a-research-paper-from-the-author

Sometimes, the authors have their own Github pages and posts their codes there. You could search in github with the author's name or by the title of the paper. If you're lucky, you can get it there.

Hope it helps!

Also try out https://paperswithcode.com/

It is the best solution I know :D

I am looking foe the code Channel estimation for intelligent reflecting surface aided multi-user MISO terahertz system

This website is also providing link to github source code

https://researchcode.com/