Reddit mentions of Morton Sodium Free Salt Substitute - 3.12 oz

Magnesium & Calcium along with potassium and sodium. This one has sodium, potassium, and magnesium and for $6, isn't bad. It's the one I use instead of salt on my food. I figure it may not be the ideal amount, but at least it's something. That may work for you if you want something simple.

This is pretty much everything you need as electrolytes go (and if not, please correct me) for a tenth of the price of Pedialyte and with better quality ingredients.

Anything with sugar or calories would break a fast I think.

EDIT: Added additional information and another link

In the US salt substitutes containing potassium chloride are the popular choice for making ketorade. If they're unavailable in the Netherlands you can get some on Amazon, it looks like they ship to your country:

https://www.amazon.com/Morton-Sodium-Free-Salt-Substitute/dp/B00XMZ0S9S/ref=sr_1_3_a_it?ie=UTF8&qid=1526738236&sr=8-3&keywords=salt+substitute

I use 1/4 teaspoon of the mentioned above salt substitute daily in my ketorade. I don't add regular sodium chloride (table salt) to my ketorade because I like my food salty, but you can throw in 1/2 teaspoon into your mix if you need. Some plp add magnesium citrate to ketorade, I use a magnesium supplement in a pill at night because it helps me sleep better.

I'd make sure your kidneys are healthy before you start taking potassium, but that's just the RN side of me speaking ;)