For what it's worth, I have over-wintered plants here in the ground by putting a tomato cage around them, and then filling it with leaves as insulation. There is a very big difference between my zone and yours, though. Perhaps you could over-winter a division inside as insurance?

If you are near Sacramento, try Green Acres nursery, one on Jackson highway between Watt and Howe, and one up in Roseville on Galleria. Bushnell Nursery in Granite Bay, on Douglas should have them too. If you're closer to the Davis-Vacaville area, Lemuria Nursery in Dixon usually has a selection of them (and would probably be cheapest). Try the Chocolate variety for a dark, Pink Stripe and Apricot Queen are also very pretty, but lighter. The ones that are mainly yellow tend to burn in full sun in the Valley. For a nice photo collection of Phoriums, the San Marcos wholesale nursery website is the place to go. (They're probably where the ones you buy retail came from). Here is a link that might be useful: san marcos wholesale nursery website

Use Osmocote.It's a time release and I've had great results with it on all my plants and my "Sundowner" Phormium has tripled in size this year.Mix some in at the bottom of the hole and mix some in the soil on top after planting.It's triggered when you water.

There was no frost at all in January in Portland and then February had below 20* temps. I am still seeing Trachycarpus and Phormium around town. Not noticing death or damage. Just drove past three taller than 10' windmill palms yesterday. They looked fine and were in an open location. I had a early blooming Rhodie suffer frost bite on all buds and new leaf tips are damaged too. I am worried about a couple hardy fuchsias planted in the fall and not properly mulched. The rest are showing growth from the roots. My newly planted (late summer) Hebe 'Red Tip's all made it with no protection. I was actually in Seattle on business when the Arctic Blast hit. I am not one who pushes the climate envelope so my experiences aren't that useful to those who do but I was just mentally commenting on Phormiums and Trachycarpus yesterday after seeing healthy ones. I grew up and learned to garden in a warmer climate so I enjoy growing the typical Northwest plants here. Whenever I've pushed the climate envelope Escallonia, some Hebes, Pittosporum, they have failed. Eventually we get a really cold winter.