You'll be hitting here at a time when area nurseries are at their least attractive and with the shortest supply of plants. While the better (and larger) retail nurseries remain open all winter, the weather is seldom conducive to nursery strolling or planting and most are reluctant to bring in much that is liable to be damaged with January/February cold weather, which can be unpredictable at best.

Wells Medina (Bellevue, just across Lake Washington from Seattle) will have their usual excellent selection of conifers.

Swanson's (Ballard, neighborhood in north Seattle) is now mostly under glass and should have a fair selection of broadleaf evergreens and perennials, specially those offering winter interest, and a good selection of houseplants/indoor tropicals.

Emery's Garden (Lynnwood, about 20 minutes north of Seattle just off I5) will have conifers, plants for containers, some seasonal color and a pretty decent selection of hellebores and heucheras.

Molbak's (Woodinville, at least a 30 minute drive east of Seattle, perhaps more with traffic) offers a spectacular selection of indoor plants. Not overly impressed with their nursery stock even in season, so have no idea what they will offer out of season.

And if you make it in December, all will be showing their Christmas offerings - trees, greens, wreaths, decorations, gifts, reindeer, etc.

During midwinter Molbak's would probably be most likely to provide the most rewarding experience.

Here is a link that might be useful: Molbak's

This might be of interest.

Here is a link that might be useful: Specialy Nursery Association of Western Washington - Specialty Nursery Guide