I have a piece of Amethyst with a diode on bottom next to my computer - have for probably 9 years now. I'm trying to remember where I ordered it from though... I'm sure you could Google it and find some places.
Negative ions are a great way to deal with the effects of EMF. I however do not generally recommend any sort of electronic ionizer. They are great in that they put out lots of negatives ions, but they also put out ozone, which many consider to be hazardous.
A natural ionizer is indeed a salt lamp. Himalayan salt is considered the best kind and you commonly find it from either the Poland or Pakistan side of the great mountain range. Also, if you want what many consider to be the best consumption salt, Himalayan salt can't be beat. Essentially it's sea salt (with all the minerals the human body has/needs) but it's millions of years old and doesn't have the many years of pollution commonly associated with sea salt.
If anyone is interested in either consumption salt or a salt lamp, I have a few clients here in the US and one in Canada that sell a wide variety of them at great prices. Feel free to PM if you want links to those sites.
And if you Google 'salt lamps' it's best to compare lamps by weight, not heights to ensure you get the best value.
For a small desk side lamp I would be sure you have a 15 watt bulb in the lamp and for larger room sized lamps, a 25 watt bulb is best. Some places sell them with 7 or even 4 watt bulbs and they may be fine purely for a night light, but they are ineffective as ionizers as it's the heat of the bulb that generates the ions - and anything less than 15 watts just won't produce much heat.