How to Find Quotes to Use on Greeting Cards

Dear Rich: I have created a card line from my art. I want to use inspirational quotes. I was able to contact Mary Oliver's staff and they said no. Anais Nin's estate want money to use them. Other than those two I am having a hard time determining. Rumi? Rilke? Movie quotes? Music quotes? Others I haven't heard of before but have lovely things to say. Can you help me sort it out? First, the animated GIF issue. As regular readers may have noticed, we've settled down quite a bit and we seem to be finding our pace. Thanks for your helpful comments and feedback.
Right, you had a question. Attempting to negotiate permission from famous poets or songwriters (or their estates) can be expensive, frustrating and time-consuming. If we may recommend an alternative, legal course to pursue -- find similar writers whose works are in the public domain. One way to find out what potential consumers might like is to use Google's new Knowledge Graph feature. If you type Mary Oliver into Google, for example, a box appears on the right of the listings (see above) showing you what Oliver-searchers are also searching for (surprise - Jalal ad-Din Muhammad 'Rumi' is No. 1). How do you know which writers are in the public domain? This article provides the basics and if you need more help, consult Steve Fishman's helpful book.