So i read on Wiki that you can buy the Katana at:

http://fallout.wikia.com/wiki/Mick_%26_Ralph%27s, sold by http://fallout.wikia.com/wiki/Mick_(Fallout:_New_Vegas).

http://fallout.wikia.com/wiki/Novac, sold by http://fallout.wikia.com/wiki/Cliff_Briscoe.

http://fallout.wikia.com/wiki/Silver_Rush, sold by http://fallout.wikia.com/wiki/Gloria_Van_Graff.

http://fallout.wikia.com/wiki/Gun_Runners_(Fallout:_New_Vegas), sold by http://fallout.wikia.com/wiki/Vendortron if Mick, Cliff Briscoe, and Gloria Van Graff are dead.

so iam at Level 39 and i chekt multiple times at Gun runner and Novac,but they never have 1 for sale(ps i killed Mick&Ralph,becuase they dont sell me anything anyway+Killed all from Silverrush)

maybe i should Kill Cliff Brisco and checkout if its avalable then at Gun Runners,but that would be a loss,because he scared sjitless of me ,every time i come there he say`s"Please dont kill me,i give you a nice prize/discount" wich he does also