just wondering if there's a site/place where users are putting any custom levels they might have developed and posted that someone knows about?  

actually I'm particularly interested in one with a building that 10+ stories high....

There is no centralized sharing hub, but you can some levels posted in this thread:

https://itch.io/t/342477/custom-map-sharing

Also there has been some levels posted earlier in the official RCGroups thread:

https://www.rcgroups.com/forums/showthread.php?2394419-FPV-Freerider-Quadcopter-...

There is also a facebook group where there are some levels available I believe (I don't use facebook myself)

https://www.facebook.com/groups/670188586415415/

I have also seen some people posting levels on Youtube.

thanks! - I couldn't see a way on the level editor to select multiple objects and duplicate these?  did I miss something? e.g. create a line of walls at ground level and duplicate them all at once to the level above them