I might be inclined to postwork the crowd in.

That is, make your baseball stadium / basketball arena foreground with nothing in the background, render as PNG so it will have a transparent background. 

Then find a non-copyrighted seated crowd scene on the Net. If it's only for your own enjoyment and not for sale or distribution, it doesn't matter if it's copyrighted or not. Better yet, shoot your own crowd scene at a local game. Apply Photoshop filters or actions so that it looks similar in style to your render. And drop it behind your rendered foreground.

Just one idea.