They can't charge you anything until you sign as a professional. Rates are negotiable but must fall within the range set by MLB/MLBPA.
You get one by knowing people or being approached by one if you are good enough. In the rare case you are good enough but don't have an advisor, Google works. There are trustworthy local guys and smaller organizations all the way up to multinational, multisport outfits like Relativity, CAA, Boras Group, etc.