To answer your question above, AutoHausAZ is on my list of on-line vendors that I use. "Wrong size" is kind of a straw man argument so that the mechanic can make a few extra bucks by also selling you the radiator itself. Many independents won't let you bring your own parts anyway. I think $390 is quite reasonable for the job.
Two issues: brand of radiator and brand of coolant. There was, maybe 5-10 years ago a rash of defective radiators by one brand, but it the blogosphere seems to have quieted down on that problem now. You could search, especially on roadfly, if you care to avoid that brand. Coolant brand selection has a bit of religious fervor to it, almost like choosing oil brand/viscosity. I use the costly BMW stuff, but they are not the only company selling aluminum engines/radiators, and many people pooh-pooh the supposed additive exclusivity.