Go to a web site such as lovingyou.com. I've included the link in the Resources section below. This is one of several sites on the internet that offer a collection of love letters to browse through for free. By browsing these letters, you should be able to find some good ideas for letters of your own.
Ask your family. You'd be surprised what people hold on to. Your parents might be holding old love letters from some of your relatives that have passed away. These can be great sources of inspiration, because years ago, there were no Hallmarks or other mass greeting card companies, so people were forced to think about very personal ways to put their own thoughts into words. Also, you might find that your ancestors' words more closely resemble the feelings that you have.
Search the web on Google or Bing for phrases that combine the words "love letter" or "love poem" with some of your favorite things about your lover. This type of search is much more likely to return a set of more personalized ideas for love letters. Your searches will be phrases that look like these examples: "love letter beautiful hair" or "love letter blue eyes."