There used to be a guy in Australia, I think, that combined a Ruger magazine and a Desert Eagle 8-shot magazine. He had some vids on youtube but I can't find them now. If you took the guts out of the Ruger mag and used some epoxy putty you could probably make your own.
Another discussion about such.
http://rugerforum.net/ruger-lever-action/3488-96-44-9-round-mags.html
8 round Pro-Mag.
http://www.midwayusa.com/product/294518/promag-magazine-magnum-research-desert-eagle-44-remington-magnum-8-round-steel-blue