You can buy kits to relocate mirrors, but then you've got to switch back and forth or have mirrors that you can't see when the doors are on.
I devised some easily removable mirrors that do the trick for me on the days when I'm running sans doors.
Please note: I cannot guarantee that these mirrors would be 'street legal' and prevent you from being ticketed for not having sufficient mirrors.
Step 1: Materials
Mirror (purchased at Wal-Mart - this one was $2.60. Be careful not to get one that is too convex or everything will appear too far away. You could use a flat mirror as well here)
Metal Straps - these are typically used in deck building. Very cheap at any hardware store
A pair of scissors is helpful as well.