does anyone have or know someone who has a boxing rink that also has space for other characters?
^^^check these links and see if they have what you want. 
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its for and underground fight club
How about this one?
its good but its to short and when put the width in the right spot it does let me upload it i think its the back ground
Click the image to enlarge and click download below it and see if that fixes the size issue – if it doesn’t there are websites to resize images ( I can’t think of one off the top of my head but a quick google search will find them.)
You can also check out freepik.com or request a background from an art shop here (in the Art Resources section) there are a ton of talented creators here who help people for free.