Is a dictionary of philosophy necessary for reading Schopenhauer? I heard that he refused to use jargon and tried to write very clear.
Try to read the book on its self.
If there are some unknown words, try to look it up on Oxford Dictionaries.
Me as a beginner in the philosophical literature, it helps a lot.
But you have to always count with the fact, that there might be some words, that do not officially exist or are a little bit different in the context of what you are reading.
The summary is, you will find most of the words there, but not all of them.
I recommend on the technical side:
Historical Dictionary of Schopenhauer's Philosophy (Historical Dictionaries of Religions, Philosophies, and Movements Series) David E. Cartwright ISBN 10: 0810853248 / ISBN 13: 9780810853249 Published by Scarecrow Press, 2004
Easily available in US & UK. The Oxford English Dictionary in any of its versions should be adequate on the non-technical side.