The Binding of Isaac Merchandise
The Binding of Isaac: Four Souls
The Binding of Isaac: Four Souls (or just "Four Souls") is the official TBoI card game. The game is for 2 players or more, and is about deceiving others, kill each other, looting, gaining items, and winning souls.
There is an official website here, which shows everything you need to know:
Maestro Media
There are a lot of The Binding of Isaac merchandise, but the most popular is from the Maestro Media store. The store includes plushies, figures, and a Binding of Isaac card game. Maestro Media isn't just a Binding of Isaac store, it also has Hello Kitty, Sally Face, and many other things.
Youtooz
There was a collaboration with The Binding of Isaac, and Youtooz. Youtooz is mainly a vinyl figure company, which do collaborations with other companies, or people. The first two figures were Isaac, and ???, with Guppy and the Satan Statue being two new ones.
These are a limited run, so they could be out.
G Fuel
Another collaboration with The Binding of Isaac is G Fuel. G Fuel is an Energy formula, which you put a spoon full into water. The first flavor is "Isaac's Tears", which is flavored "White Grape Pineapple 😭", and the second flavor is "Brimstone", which is flavored "Cran-Raspberry".
These are a limited run, so they could be out.