The hero3 black has the better quality but the hero3 silver is still an awesome camera with all of those features minus some of the quality (black is 12mp, silver is 11mp etc.)
I would recommend getting the motorsports edition because of the mounts it comes with
Can't really tell you the best place to get one, they're mostly all the same price
You might be able to get one cheaper off of eBay or craigslist