Antonis has spoken well, we are working hard on providing many audio assets with the release of Echotopia.
In fact, we want to provide enough for any user to be able to create the basic soundscapes needed for any scenario. That's why we created the "Echotopia Content Requests" subforum. You can use that to ask us about your needs, so we put your wishes in our ToDo list
YouTube's audio library is also a good place to find some content and if you also search YouTube for ambience loops or specific soundscapes, i.e. "forest soundscape loop", you will find many videos which then you can extract the audio using some free tool to extract audio from video files. Many of the content there is published under public domain or creative commons licenses that allow the audio to be used under the various usage scenarios Echotopia users may have, so check that too before ripping content from the internet.
Apart from the "factory-included" content we are developing, there are many resources like YouTube, FreeSounds, etc. Admittedly, those sources provide audio assets of questionable quality and many times questionable intellectual property. Sometimes you might find that a sound that says "use it freely" is actually put there by a person that pirated that sound and presented it as "free". This is a problem with content creation using questionable resources and you should be careful especially if you are planning to use your soundscapes publicly on broadcast streams or content/product distribution, or in games, films, VR experiences, that will go outside your computer.
That is why we develop our own content and I urge everyone to give us ideas and requests in the appropriate subforum I mention above.
Soon we will start to roll out content for the Insiders and after that, we will have the complete factory library ready, so now it's a good time to gather ideas and requests.