Blue Nile offers a free ring sizer at bluenile.com Fill out the form at bluenile.com/ca/education/rings/find-your-ring-size to request your plastic ring sizer. It will be sent through the mail so you will need a mailing address.
You can also download a free ring sizer guide in pdf. Visit bnsec.bluenile.com/bnsecure/assets/chrome/pdf/ring_sizer_2016_US.pdf to get yours.