Take out dim sum in Valley or Valencia?
A&W in Northridge (Nordhoff and Prarier) will do takeout with everything on their dim sum menu. I don't know if they will take phone orders, but if you go during the dim sum hours -- 10-2 -- they will just pull things off the carts. It has the best variety of dim sum in the valley.
BBQ Unliminted has a few items, but not a lot of selection. 99 Ranch also has some dim sum in its hot deli -- usually shu mai and har gow. There are some buns in the refrigerated section.
You can get SOME dim sum at BBQ Unlimited, on Sherman Way. You may be able to get take out at A&W Seafood on Reseda north of Nordhoff. And there may be dim sum at Sam Woo at Victory and Sepulveda. Those are the three places in the Valley that have any kind of dim sum at all, and none of them have the variety or quality of even the Chinatown places.
Your best bet, though, may be to buy frozen dim sum at 99 Ranch (next to Sam Woo) and steam at home.