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These are a few places where you can download Sentai torrents:

http://mishicorp.net/community/index.php

Mishicorp. They were subbing Mahou Sentai MagiRanger, but they gave up on the project. You can still get the episodes they've done and some other cool stuff (soundtracks and such) there.

http://www.tvnihon.com/

TV-Nihon. Probably THE best source for not just Sentai, but all Japanese action television. They have the first three episodes of ZyuRanger here, for you PR fans that are interested in the Sentai the original show was based off of.

http://anarion.cjb.net/

Anarion. Loads and loads of stuff here, but it's all in RAW (non-subbed Japanese). I'd still recomend going here, a lot of this stuff transcends language anyway.

Those are the ones I use, personally. Feel free to post your own sources.

For the Technically Inept such as myself I would suggest...

www.youtube.com

They have a lot of streaming videos you don't have to download a lot of extra stuff for. However they don't have that many episodes of sentai.

I beleive you can find the whole series of live action Sailor Moon, if you type the tags right.

Someone on youtube, has been kind enough to start uploading Kamen Rider 555(or faiz) and the first dozen episodes of Magiranger.