Where do we get the belief that Mary was raised directly up to heaven, foregoing death?

The belief in the assumption does not necessarily mean that Mary did not die.  The Church has never categorically answered that question.What the Church HAS said is that at the end of her human life she was taken body and soul into heaven. 

Personally, I believe that Mary DID taste death. Since Jesus Himself experienced physical death it seems odd to suggest that Mary was somehow exempted from that normal human fate.  But death was only fleeting, for she was taken up into Heaven where immediately she experienced the resurrected life that will be given to all believers at Jesus' second coming.

For more information on the early writings concerning this subject I direct you to the following link:  http://www.newadvent.org/cathen/02006b.htm.