Are there any tool rental places around your area? That is a typical tool to have available at the rental places just for the reason you mentioned.
If you're just trying to pop the taper loose in a steering knuckle, buck up on one side with like a 2# sledge and smack the opposite side with the same size hammer. On a good, really lucky day the jar will pop the taper loose. On the rest of the days you'll pound 'til your eyes cross to no avail.
