http://www.psypokes.com/rs/feebas.php

In Black/White, there is absolutely NO way to get shiny reshiram/zekrom. But for others, you can do any amount any time, you have the same chance every time you SR.

Regarding Feebas, boil a cup of coffee and be prepared to spend a few hours getting a hold of the damn thing. You'll need a lot of patience. Or maybe you'll get lucky and get it right away, who knows.

It doesn't matter. I still recommend RGN abuse if you want to stay legal.
I don't know much about feebas though, got mine from a friend back then.

is rng possible on white too? and a guy told me, that it is not possible on a dsi. is it true?

Its sure possible on white, and RNG is sure possible on DSi, but there are two kinds of RNG-Abuse, with C-Gear(hard) or without (easy), and im not sure if the easy one works on DSi. But to RNG shinys you need to get your SID, which you can either RNG when starting a new game or viewing it with an AR or via Pokegen (if you really want this)

could you shortly explain how to do that c gear one and what is SID? thanks, iam really new to this

No I can't actually, because a short explanation is still like a half page and I never did normal C-Gear RNG because I have a DSlite. I'd recommend a Guide on Smogon.com

A S(ecret)ID is a hidden number which is set when you start a game. Together with the ID which you can see on your Pass, it determines wether a pokemon will be shiny or not.

well, it looks like a damn difficult thing