

Tuna and large fish were traditionally used for “garum”, while small fish were used for “liquamen”. (Though the naming changed with time to reflect production processes rather than raw materials).
Garum was always expensive, while liquamen was initially cheaper and the byproducts of garum production (muria and allec) were the cheapest of them all, and probably the latter was what was given to slaves.
It is available in some specialized supermarkets, I think at least one Spanish version is available in the US. In fact, there are several versions available from the same country, but with modernized recipes.
If you can find a bottle, here’s a recipe for you: https://www.historicalcookingclasses.com/garum/
In fact, some high-end places tend to fabricate their own garum with different bases than fish: https://www.foodandwine.com/condiments/why-chefs-have-loved-garum-since-ancient-times
And if someone didn’t get enough sauce out of my comment, here’s some discussion on what to call them: Garum, Liquamen and Muria: A new approach to the problem of definition