What is the next-best alternative to actual sun in producing vitamin D?

For those who live in the Northwest U.S., what is the best way to produce/synthesize/metabolize (I always forget which one it is) vitamin D during the winter months? Supplements don’t work as well, so I’m specifically wondering if anyone has any suggestions as to special lamps, tanning centers with particular booths that are safe, etc. Or perhaps monthly trips to Arizona should be in order?