Hey @trgiezhang ,
I think what you are looking for is a bottom bracket cable guide. ( I think shimano calls it SP-18-T) REI shops probably have them as a miscellaneous part, but I don't think we sell it on line. I am sure if you looked at Jenson USA or Nashbar websites they would have them listed. I think they are $2 - $3. so normally i will buy a couple and throw them in a drawer and forget about them. Its funny how the small common parts are sometimes the hardest to describe. Hope this helps!!