We sell both the raws chemicals you need for the Cyanotype Process. You can also get pre-coated paper that is ready to use. Alongside this we sellPotassium Ferricyanide is a raw chemical and one of two main ingredients that make up the cyanotype process alongside Ammonium Ferric Citrate.
Another use is use it to make up bleach for use on prints or negatives.
Measure 25g of Ferric Ammonium Citrate and add water to make it up to 100ml.
Measure 10g of Potassium Ferricyanide and add water to make it up to 100ml.
Make sure you fully dissolve both the chemicals.
In subdued light, mix the two solutions together.
This formula will make up 200ml of working solution, enough to coat roughly 50 sheets of A4. You should coat paper away from sunlight and leave to dry in the dark.
For full instructions see This is How You Make Cyanotypes.