You've seen the anime and want more story ?
You want Sayaka and Madoka to survive ?
You want to know Mami's past ?
Well, a solution exists, and it does not include a contract with Kyubei.
Introducing to you....
[Only admins are allowed to see this link] !!!
It's a PSP game in which you have different timelines Homura has lived before the one from the anime. Each timeline is on one of the girls (Madoka, then Mami, then Sayaka, Kyoko and at last Homura) and there's an extra timeline. At the end of each timeline, there are a few possible ends (either a good one, or a bad, bad one, the worse being poor Sayaka of course u.u)
Of course, the game is in japanese BUT some nice guy from Youtube has translated the endings !!! So click the link above for the written thing, or
[Only admins are allowed to see this link] for the video translated =3
Warning : Sayaka can have an even more cruel fate than in the anime =/
- Spoiler:
(and now I've seen it, I'll have to explain why Kotaro's sister's body does not start to rot ^^')