Its easy to make your own topdown tileset

Open this image with GIMP

Create a gradient with blue (water color) on the left, brown (earth) in the middle and green (grass) on the right.

To create a gradient go to the menu Window->Dockable Dialogs->Gradients and then create your new gradient.

To apply your new gradient, go to Colors->Map->Gradient Map

and viola, you have a tileset with all permutations of left to right, top to bottom etc. tiles.

Just cut out your tiles in 32x32 pixel tiles.

It initially creates a rather dull tileset, so you might want to play with adding noise (filters->noise->pick), blurring etc before applying the gradient.