How do I find these?

Hey this is kinda a dumb question but how do I find these registration numbers? Thanks.

In general, foot-high numbers/letters painted somewhere the rear of the aircraft.

well yeah but Im talking about a website like planespotters.net, are there any others?

http://faa.gov/
http://airfleets.co/
http://airliners.net/
http://jetphotos.net/
http://acarsd.org/
http://ibhomeradar.org/
http://flytecomm.com/
http://laasdata.com/
http://www.joebaugher.com/usaf_serials/usafserials.html

and many more I am sure :cool:

For Canadian aircraft, except military
http://wwwapps2.tc.gc.ca/Saf-Sec-Sur...uicksearch.asp

Thanks guys