How to Download Catalina 10.15.0

My MacBook is currently running High Sierra 10.13.6. I haven't updated it since this release as there was no need based on what I use it for. Apple are discontinuing support for EA's Origin App so I need to install the EA App to be able to continue to play The Sims. The EA App requires at least Catalina 10.15.5. I tried to download and install 10.15.5 but was unable to without first installing Catalina 10.15.0, which seems to no longer be available. I am not interested in installing any later OS than I absolutely have to. My MacBook is an old Air and is working fine, I don't want to overload it with a recent OS that will break the good thing I have going. I suspect that Catalina might be the last compatibile software for my computer but not sure how to check which model I have?

Does anyone have a link for Catalina 10.15, or another solution, so I can continue to play literally the only game I like.

Thanks