I bought some of these NOS mains filters from ebay for a very reasonable price of 11 each.

I have been using then to provide some isolation of SMPSs from the mains and they seem to work rather well. Opening one up revealed very high quality components inside and very good build quality too.

Really surprises me what a difference isolating SMPSs has on the system. Just shows you what they must throw out onto the mains and how it effects other equipment.