We've had good expériences with etsy and spoonflower.

I've gotten all mine from wallpaperdirect.com

You pick a wallpaper to love for a long time. The labor to hang it doesn't vary- may as well hang something you will enjoy for years.

"Building" is too far ahead to even worry about the wallpaper, unless you have every last detail figured out. There will always be a paper and there will always be a source.

When you decide exactly what room you want to paper, exactly the feel you want in a space? You search that.

A great , skilled paper hanger ? That's the key to ANY paper: )

IMO wallpaper never gets chosen before decor is in place unless you plan to let the wallpaper drive all the other choices. As for where alot of that depends on your style , where the wallpaper is going , just way too soon to plan