Mushroom Ketchup is not actually ketchup in the way Americans traditionally think of it. It’s more similar to a Worcestershire sauce that’s been flavored with mushrooms. It’s very popular in England (and extremely so in the 1700’s) so finding it in the US can be a bit of a struggle. Amazon sometimes has the most famous brand, Geo Watkins, in stock; but more often doesn’t. I personally was able to find it at Jungle Jims in Cincinnati, which has a very large international section. The recipes this ingredients is featured in (Mushroom Cappuccino & Mushroom Parfait) both take a lot of time to make giving you no excuse that you didn’t have time to locate the ingredient.