I've checked the website you recommended. It offers a look up of registration numbers for US domestic flights (main carriers only) using the US Bureau of Transportation Statistics data which is available directly from the BTS, so that's not really exciting. I tried a couple of international flights (originating in the US or outside the US) and got the following message "Inernational [sic] routes are only available for First Class members. Please click the star up top to learn more." So, I clicked and learned that I can get a membership by providing a credit card number for $2.99 a month. Has anyone tried that? Is there really any data available for those with membership? It is unclear whether one can sign up for just one month or how to cancel.

Thanks.