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/
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: