find soundtrack with the cheapest licence
As far as I'm aware. There is no 'soundhound' or Shazam type app for royalty free stock music. Usually these websites require that the owner only sell the music on their website rather than a competitors. I'd say the likelihood of finding the same song another site is slim. 

With that being said for cheap stock music I always go to audiojungle and for the higher budgeted jobs I go to musicbed. Sometimes I will check out extrememusic just to establish what the client likes; but that sites tends to be studio-only unless you are willing to fork out $$$.