I recently managed to create a disk image of what was apparently the last known copy of P-Lisp on planet earth. P-Lisp was a 1981 implementation of Lisp that ran on an Apple II. The disk image itself is here. The surprisingly painful story of what it took to get the image is here. And last but not least, documentation can be found here.
If you don't have an Apple II there are a number of Apple II emulators that you can use. I like Virtual ][.
I used ADTPro to create the image. Highly recommended.
If you actually want to learn Lisp you should not try to do it using P-Lisp. It is quite dated, and mostly of historical interest. Try CLozure Common Lisp instead, and one of these fine books. This one is particularly good, and available for free on line.
Finally, if you need a USB/RS232 adapter for OS X, you should avoid the IOGear GUC232A like the plague. The IOGear drivers are perniciously buggy. I had much better luck with a no-name adapter and the Prolific drivers.