You can do a bit more manual research by going on Amazon, Etsy, Walmart, any retail site, for that matter, and search for top sellers in your niche. Look for products with word-based designs that have hundreds or even thousands of reviews.
Let the data tell you exactly what people are responding favorably to. There are paid tools that aggregate all of this info, speed up the process, and make it look extra pretty.
Whatever your preference, you should be able to create a substantial list of viable ideas in a relatively short amount of time. Now, the ethical thing to do would be to come up with your own slight variations. Don’t just copy other people’s words verbatim. Good news is, it’s not hard at all. Don’t overthink it.
Just look up some synonyms, change a word here and there, and you’ll be fine. No need to second-guess.
Keep your eyes open for sayings that could work well in other niches too. Rachel was once searching for witty lines for teachers and came across a T-shirt that had: “‘I do this for the money,’ said no teacher ever.” Clever, right? So she applied it to firefighters: “‘I do this for the money,’ said no firefighter ever.” Talk about an easy way to scale up your print on demand business.