Can i get just any power supply for my computer?

I have a brand new dell inspiron slim that needs a graphics card, but first it needs a power supply, so i was thinking about putting in any 450w good quality power supply to it, would it work? Does it have to be inside the computer? might not be big enough if it does.
Update: I dont really mind it being outside, its just that its the slim tower and i dont know if it can fit.
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