I know a few people who do get Madrones to grow here, but I have never had much luck with getting a 5 gallon sized tree to actually survive and thrive. They are available from growers such as Suncrest Nursery out of Watsonville, Berkeley Hort Nursery could order them in. I'd suggest waiting to plant until late October or November, and maybe start with smaller one gallons. I would probably plant 2 to 3 times as many as you actually want, expecting some losses along the way. Getting them established during late fall/winter when you don't need to water is more of a sure thing than planting during the dry months, they can be very touchy about summer watering.

Alternatively, had you considered the similar looking, but much less sensitive to diseases, Arbutus 'Marina'? I love using this tree, and it looks very similar to our native madrone. Other natives that you could use as very large shrubs/small trees to about 18 feet tall might include our native Toyon, Prunus illicifolia or Myrica californica. All would do well with dappled light under redwoods.

The trouble with A. menziesii is that for some reason it transplants very poorly. I've planted several from 4" liners or 1 gallon containers (mail order, from Forestfarm) and never lost one. But I suspect if you buy a larger size, you'll lose them.

And it's a terrible idea for a screen, because it grows incredibly slowly, only a few inches a year. So your grandchildren may inherit a stunning screen of madrones, but you'll never see it.

I second all of bahia's lovely choices for alternatives.

My native woods contains lots of Arbutus menziesii which in a setting with other trees tends to grow tall and sparse and nearly always far out of plumb. The Marina are a much better shape and easier to grow as well. I also like bahia's alternatives. Al