Some online stores sell it, try an Etsy. Otherwise it's easy enough to make some of your own.
1 teaspoon powdered iron oxide (Rust)
13 teaspoons clear alcohol
1/2 teaspoon vervain herb
1/2 teaspoon blessed thistle herb
3 teaspoons powdered gum arabic (Can be bought from art suppliers as a liquid, poured into a plastic tray and left to dry hard. Then simply break into bit and grind to a powder)
Put the iron oxide in a jar with a lid and add the alcohol. Cap the jar and shake now and then to dissolve. When dissolved add the herbs and leave to steep somewhere dark for a week. After then strain the herbs out, add the gum arabic and shake again until the mixture has thickened. Store in a jar and shake before using as it may separate while stored.