Which one is best? Don't ask, you'll get a different answer from every person. Evox (Evolution X) is the one most people start with, and it's the easiest to set up in many instances. The easiest way (not the best, but the easiest
is to get a copy of Slayer's Evox Auto-Installer Disk (version 2.5 or 2.6).
But any dash will work. Just most people start with Evox, then experiment with other dashes as independant apps, then choose a favorite and use THAT as their boot dash. Personally, I use Xbox Media Center, cause it kicks goddamn ass, but that's my opinion. Also, it's not the easiest dash to start out with or setup up as a main dash (for an unaccustomed user).
As to where to get them, we can't tell you here.
You will find the phrase "the usual places" a thousand times on these forums. Start looking there. You could search the web yourself, but unless you get them from an official source you could be flirting with problems, incomplete builds or, worse, viruses.
So, how about starting with Evox, from Slayer's if you want a firm starting point.