To find birthday messages online, you can visit websites like Hallmark, Blue Mountain, American Greetings, or 123Greetings. These platforms offer a wide range of birthday messages categorized by age, relationship, and tone. Simply search for "birthday messages" on these websites to explore various options and choose the one that best suits your needs. Additionally, social media platforms like Pinterest and Instagram also have numerous accounts dedicated to sharing creative and heartfelt birthday messages that you can browse through for inspiration.