Garr Reynolds compiled a list of several resources to find quotes for presentations. Reynolds is the author of Presentation Zen, a blog you should follow if you create presentations with any frequency.
He also offers a few tips on how to (and not to) use quotes.
A couple tips I'll add:
- Don't feel like you must precede every section with a quote -- it's okay, and can have more impact, if you just do it once or twice.
- Relevance is key. People get bored and gloss over on the quote thing if they aren't spot on, and spot on is hard to find.
- Offering a pointed quote at the end of a section or subject can serve to emphasize it, or it's importance, nicely, and can be more effective than placing the quote at the beginning when the reader/audience doesn't have a context (yet) in which to apply the quote.