How different is the mineral content in distilled and demineralized water? I would like to get some demineralized water but I can't find it anywhere. So I wonder if I should settle for distilled.

it depends on how it is treated I suppose. look for suppliers and ask them, they will tell you far more accurately than anyone here ever could.

As far as I know, the only difference is in the terminology. It is easily available though. Demineralised/distilled water is available in all chemists, and even car-parts shops (for battery top ups). If you want controlled purity (e.g. for medical use), go to a high street chemists. They all sell it; small bottles to 5L containers, and will be able to give you specs on it.