I think the key is going to to find somewhere you can buy a good amount in bulk (you'll save more money). You're probably going to need a lot of chocolate to make 600 truffles (I'm guessing around 15 lbs.)
I know that Belgian Chocolatier Piron in Evanston sells large blocks of the chocolate he uses (Callebaut, I believe). I'm unaware of his pricing.
Honestly, if it were me, I'd probably source it online and have it shipped. Callebaut would probably be my first choice based on price and quality.
Belgian Chocolatier Piron
509-A Main Street
Evanston, IL 60202
847-864-5504
http://www.belgchocpiron.com/Best,
Michael