The SDS for Barnacle Buster identifies phosphoric acid as the primary ingredient at 85% at a concentration of 5% - 20%.  I think the rationale for not being specific to try and protect trade secrets.  A similar product is available at home improvement centers as a phosphoric tile and grout cleaner.  Do a little napkin math and adjust the concentration & you could come up with a far less expensive alternate. 

If you do this always add acid to water NOT the other way around.  An exothermic reaction may occur causing a violent reaction if the solution is mixed improperly.