You can get spacers that put one foot out ~25mm more than the other, but those are probably serious overkill. I've got a similar issue to you, only for me it's my left foot. Here's my solution:
1) Use Look Keo pedals. SPD is not the most adjustable system, due partly to the limitations of the 2 bolt attachment. I can move my 3 bolt road cleat around a lot more on the bottom of my shoe, so can move one foot out more than the other.
2) Put a washer or two on the thread of the pedal you want to space out - not enough to compromise the attachment to the crank, just enough to get an extra few mm of spacing.
3) Try some of the mouldable footbeds made by e.g. Specialized and Bontrager. If one foot wants to go out more, it may be due to a fallen arch or similar and extra support can help with this. You might want to go to see a podiatrist/chiropodist about that if you haven't already.