Another good source of information about rare perennials is a public library. There will no doubt be countless books about plants and perennials. The information about rare perennials gathered from library books will likely be more complete that the information gained from the Internet.
A third place for information about rare perennials would be from nurseries or garden centers. These places may not only have the information being requested, they may also have live examples of the plants. A garden center will be able to give information more tailored to the specific needs of each case, as opposed to books and websites, which cannot filter information out.
Wherever information about rare perennials is obtained, make sure that the source is credible. Many websites are not run by notable sources, and could contain information that is harmful and sometimes dangerous to a garden. When in doubt, always ask a professional that can be trusted.