Try one of these libraries:
Pipework & Structural Steel http://www.3DCADMax.com
3D AutoCAD Pipework – http://www.HerculeDesign.com
Or if you'd prefer 2D:
2D AutoCAD Pipework - http://www.3dcadmax.com/2DPipeFittingsAutoCAD.htm
Autodesk also offer AutoCAD MEP - http://usa.autodesk.com/adsk/servlet...112&id=3290681
AutoCAD MEP is the front runner in high end piping programs. Inventor Professional has a pipe program that is impressive but has a very steep learning curve if you are from an AutoCAD background.