Upload & Sell: On
Not all mirrors have the same reflectivity. No mirror reflects 100%. Some are more than others. If you find a mirror from an old camera that used to reflect 90% when it was brand new, it's not going to reflect 90% now. A cheap mirror from an old camera is not likely to be better than what you have in a stock 5D.
If you want better, you have to spend money, and have it done by somebody who makes top quality optical equipment.
+1 on this (above).
Silver has the highest reflectivity of various metal (optical wavelengths) first-surface mirrors. Aluminum is a close 2nd. Gold is right up there too. Chrome (WTF!?) is not even in the picture.
All of these have a molecular coating (think transparent anodyzing, essentially) that prevents tarnishing, when used in this way.
Edmund Optics offers custom mirrors in any size, with any of a variety of coatings. Silver or aluminum would be the obvious choices.
If I was going this route, that's where I'd start. They (Edmund) have an amazing catalog, one can learn a lot by just reading it. And there's other providers too.
Ultimately I think this quest is not worth the trouble though. It's based on the (false?) premise that more light reflected from the mirror will increase brightness in the finder. Although that might seem logical it's probably likely that it won't have a big effect, if any. My guess is that the light is throttled from the other things in the system - the focusing screen and the prism itself. Shoving more light through it is not necessarily going to result in a brighter image. (to wit: The effective "aperture" for the focus screen is about f/2.8 or so. Brighter lenses than that don't appear brighter in the finder, and stopping down doesn't darken the finder much if at all between f/1.0 and f/2.8. )
If you really want a bright finder, hack off the prism and build a little hood to use the screen as a waist-level finder, ala medium format SLRs. Probably not any more work than the above idea of replacing the mirror. Of course, a bit more permanant perhaps. ;-)