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boiling water might be the bet for neg ions or a small water fall.
Air ions

As well as negative ions, there are positive ions floating round in the atmosphere too (known collectively as air ions). Air ions occur when an electron is knocked off an air molecule (a positive ion) and attaches itself to another one (forming a negative ion). Ions are thus always formed in pairs of negative and positive ions.

Most ions are created by radioactivity and cosmic rays. They usually only last for a matter of minutes before a negative ion meets a positive ion leaving two neutral molecules again. Ions can also attach themselves to solid surfaces causing them to become charged.

In general, there is usually a higher concentration of positive ions (by about 25%) in the air. This is because the positive ions are bigger and so move more slowly, hence diffusing (eg. into charged surfaces) more slowly than negative ions.

Concentrations are high in mountains (2000 negative ions per cubic centimetre compared to 2500 positive ions). At sea level, the figures are nearer to 1500 and 1800 respectively. After a storm, the balance may reverse sharply for a while. Concentrations inside buildings are much lower (typically 150 neg; 200 pos).

Evaporating water can cause an excess of negative ions over positive ones. This is why waterfalls and the seaside have higher negative ion concentrations.

Ion concentrations can vary over short distances, particularly indoors. You may find that one end of a room has an excess of positive ions and the other end, negative. Outside, variations tend to occur over longer distances.

It is not easy measuring air concentrations and accuracy is generally low.