Upgrade PSU in Inspiron 3020

I'd like to upgrade my psu from the 180w it came with to at least the 460w that theoretically is available, but I can't find it anywhere. I was given two different product numbers for the 460w psu, # VTG94 and # 4H6F4, but I can not find those on any psu anywhere online. Does anyone have any suggestions on a) where to find that psu and/or b) alternative compatible power supplies?

Thanks, Nick