One resource for virtual sets with green screen inserts is videoblocks.com. They aren’t free to subscribe, but frequently offer a free download period for a subscription.
You have the option to cancel the subscription; you’ll just have to be intentional about following through on that if you do not want to be charged.
I am sure others will chime in with freebie options.
System:
Dell Precision T7600 (x2)
Win 7 64-bit
32GB RAM
Adobe CC 2014 (as of 7/2015)
256GB SSD system drive
4 internal media drives RAID 5
Typically cutting short form from HD MP4 and P2 MXF.