One type of filter is available from Harmonics Limited called a "3rd Out Filter". They do have a web page. Perhaps someone else can come up w/ another. I hope I understood your question.
Be carefull before you choose a filter of any kind !!!!!!
Two of the most common problems resulting from wrong filters in the network might be :burning of the filter and vicinity and / or explosion of high voltage to low voltage power transformers supplying the network.It is crucial (!!!) before you install any solution to inspect and study your network .Being a professional in the field I would recommend first to study the network in depth. One of the other solution for harmonics may also be a transforme isolating the creator of harmonics from the network and so solving the problem in the most economic way. A.Rash BScEE & PE
a_rash@bigfoot.com