You can go their website and ask for a sample, same goes with milk. Or go to a paed clinic, some of them do give out free samples. You can check out tesco or aeon and ask their worker. I went once to tesco during baby sales, and got free samples. Hospital also give free samples!
You can go to website example drypers, there is section for free samples. Key in address for delivery. You will get in around 2 weeks.