I would argue "a bus number six" is actually wrong in english. The article does not belong here at all.
"She is looking for a number six bus" should be fine.
Going through this course I cannot help but remember the Soviet era English textbooks... Same unnatural and weird wordings.
Any reason that "She looks for bus number six" wouldn't be accepted?
Btopps, "She is looking" sounds more natural than "She looks" in English, unless the sentence begins,"Every morning...."
"she is looking for bus number 6" was not accepted :-)
Can I use "bus line 6" here?