Is a render farm suitable for using high-res stills? This question is primarily for curiosity. I'm aware that render farms are used as a method to speed up rendering time for animation sequences. Is it a good alternative to use for high-res stills that would take a lot more time to render by just using a standard computer to do so?
If there is such an option, where can it be found?
That will depend on the render farm you plan to use. Normally a render will generate a complete image which means animations can easily take advantage of multiple computers as each computer can render a single frame from the animation.

It is possible to cut up a single render into multiple pieces that can later be stitched together into a larger image, this will allow multiple computers to work on the same larger final image. Blender's render border option can be used with a python script like in this answer, it is possible to use imagemagick to automatically stitch the pieces into one image.

Some commercial render farms include an option to do this, Render.st is one example. If you are creating a personal render farm then chances are you will need to add this ability yourself.