Hello , im using alienware x51 r1 currently with gpu of nvidia geforce gt 640 and a psu of 330w external psu 

I want to upgrade to Nvidia GeForce gtx 1060 3gb . but it requires a psu of 450 W atleast.

I can't find any external psu that has 450 W , can anyone help me?

Do I really have to change my case to fit an internal psu?