not worth is... a custom pulley kit (which you would have hire an automotive engineer to design one) would cost around $2,000k+... with only gains of around +3-5hp (5hp if you underdrive the alternator - not for audiophiles). Cost vs performance is not worth it... not to mention, the average garage joe can't install them himself without the proper tools and measurement devices for proper tension.
Installation usually involves removing the radiator fan (extreme cases demands taking out the entire radiator) loosening the belt tensioner, then removing the appropriate pulley(s), then returning to tension specs as per BMW... it IS possible to DIY, but it takes trial and error to listen and feel for faults in the tension until you get it right and risk damaging your engine/crankshaft/components.
So with that said... Cost vs Effort vs Performance is waaaaay not worth it...
and this is where you would start: http://www.intent-design.com/innovative-services.php
also, i'm not sure where "the" guy bought one, but i wouldn't be surprised to think that it wasn't meant for N52. There was a rumor that the E36 3.0L pulley kit would work, but i talked to TurnerMotorSports about it and their engineer was certain that the pulley system is thoroughly different as there is no pulley-driven water pump. If you can find the article, can you reply with the link?