One way to locate a list is to rent one! Many marketers in the US rent mailing lists from various magazines (Obvioiusly, in the US) that cater to certain demographics. Look for DIRECT MARKETING and MAIL LIST CENTERS in your search engines.
Some list compilers go beyond merely magazine lists, of course -- they compile their own lists, and usually have them raked, perhaps by income over $500,000 USD, or houseowners whose homes cost over $150,000 USD ... and so on.
Current client mailing lists will, upon study, reveal the general demographic outline of new potential donors -- with that outline in mind, it's not too hard to prescreen various databases to arrive at useful new mailing lists. You'll know it's the right list when you MUST SCREEN existing donors out of it, and they're there!