Where do you usually find so-su? On top of okonomiyaki, tonkatsu, Japanese-style hamburger, yakisoba, and takoyaki, to name a few.
There's actually several different types of so-su, based on how thick it is. "Usuta so-su" is thinnest and most similar to the Worcestershire sauce that the British invent and Americans are familiar with, and the "tokuno or tonkatsu so-su" is very thick and is sweeter and less tangy than the usuta variety. The third is "chuno so-su" which is almost like a blend of the two styles: viscosity in between the other two styles, with a mix of tanginess and fruity sweetness. The type most familiar around the world would be the thickest sauce, the tonkatsu sauce.
|Three styles: thin, medium, thick*|
If you don't have access to a Japanese market, there are plenty of fascimiles you can make at home. The most common substitution is as follows:
1/4 cup Worchestershire Sauce (Lea & Perrins is the ubiquitous brand)
1/4 cup Worchestershire Sauce
1/2 cup ketchup
Additional easy fix: adding some sugar to HP Sauce or A1 steak sauce to make them sweeter.
However, if you're looking for something with a little more complexity, I recommend the recipes on Saveur and Serious Eats. Both of them go for soy sauce for added umami, both rely on mustard for a bit of a kick, and both deliver results.
But if you're lazy like me, and can't hoof it to a Japanese market, it's worth ordering the real deal from Amazon.
*Picture credit: Japan's Wiki on Tonkatsu Sauce.
Recipes that use So-su:
Holiday Ham Korroke
Yuzu Kosho Bloody Mary