The next puzzle that you should get is 2x2x2 cube as this picture below.
The 2x2x2 Cube is not easy to be found compared to 3x3x3 cube.
Most toy shops would have them but many of my friends got theirs from online stores.
Again, you have to remember that is to get the durable ones.
2x2x2 cubes are of course cheaper than 3x3x3.
You can get one from below, if you want.