I'm lucky as I wear a 9.5 in almost all shoes/boots, so I've never had a problem.
I have two friends that have big feet (~12) and they have the same problem as you. I know that Salomon, Ride and DC all 'say' they have some kind of shrinkage tech for their boots, but the tech (and size decrease) varies for each brand.
Unfortunately (for sizing issues like yours).. the only way to find the right boot is to go out and try on everything you can. Comfort > Size, but maybe you could take a small tape measure with you to test the actual footprint for different brands of the same size boot.
As for using risers... I have no personal experience, but what's more important to you?? Having a slimmer board or having more direct contact with the board?
Personally, I'd pick up the best (read: most comfortable) boots I can and then check out different boards (Wide / Mid-Wide) - and Regular because different manufacturers all have different widths - and I'd get one that allows me to use my stance without a riser pad.