Are any specific locations in Steelport specifically suited for the 50 "Emus Destroyed" challenge? Otherwise, where can I find these vehicles in abundance?
The trick to this challenge is to find one Emu, park it in your garage, and then keep pulling it out of your garage and destroying it over and over and over again. Once it's been logged into your garage, it will return there every time it is lost, stolen, exploded, etc.
There's sometimes a "vehicle repair" fee associated with bringing out a damaged or destroyed car, although I was able to do this multiple times in quick succession without ever paying such a fee. It's possible this was a glitch, however.
The Emu's not a terribly uncommon car, so driving around town you will more than likely encounter one after a while. I'm not aware of any locations where one is reliably parked (and neither is the internet, as far as I can tell...). One YouTube video seems to have a method of getting them to spawn that they claim is reliable, although I can't vouch for it:
Mascots like the Emu, and there's a particular survival challenge that is just wave after wave of mascots. Since the survival challenges are randomized, it's not the best way to find Emus, but keep it in mind if you haven't already done all the survival challenges - it might be best to wait on this particular challenge until after you've done that particular one.