I bought quite a nice Modular PSU from PC World - definately payed over the odds for it (About £90) but as you say - if you need it now sometimes the premium is worth paying.
I had a bit of a sarky conversation with the manager at the time (this was the pc-world in moorgate), I asked why for a shop called PC World there were only 12 different PC's (only one isle) on Display and only a handful of components - the rest were Plasma tv's DVD's, cameras, ipods and Consoles etc.