Depending on the type of project you're looking to buy and the amount you're willing to invest, here's a handful of marketplaces and brokers for online assets for small/mid-sized projects (with some larger scale ones):

https://www.sideprojectors.com/

https://exchangemarketplace.com/

https://www.flippa.com/

If you're talking about generally larger investments, probably walking away from the realm of side-projects, there's these few that are generally geared at more of a full business acquisition style sale:

https://feinternational.com/

https://latonas.com/

https://empireflippers.com/marketplace/

Flippa is an option if it's a smaller project [1].

If the project is moderate sized to larger, it would be advisable to speak with an Investment Banker.

[1] https://www.flippa.com/

https://www.sideprojectors.com

Happy to help and and answer any questions! :)

UpWork