Find water demineraliser for use in a steam iron

Any undersink reverse osmosis system will give you the low tds high quality water needed for the steamer. Make sure the ro system does not have a mineral cartridge to add back calcium that it removes from water.

You can use cheap pure distilled water (with no additive) as used to top up car battery: supermarkets and auto shops will carry the product.
You can also make your own by investing in a water distiller but they can be expensive and the cheaper model will take about one hour to make one liter.
As a cheap alternative and easy to find alternative, use a good quality drinking water filter (jug) to make your clean product with tap water and your steam iron will still thank you!

Hope it helps!