It’s basically just a lightweight (I think hollow) wooden cube with sanded down corners and edges, painted. Not sure how the lettering was applied. The type is very slightly raised.
In theory it would not be too hard to buy plain wooden cubes online and make one of these yourself... But it would almost surely look sloppier, and for 15 bucks it seems worth it to get it done cleanly. Vendors like this offer a fairly wide range of sizes, colors and typefaces.