Transfer function of real components
using an op amp with a GBP of 3MHz and you want to use all 3MHz


Generally this isn't done. To do it might require manually tuning the feedback circuit to optimize the overall circuit performance in some kind of test jig. But usually it's cheaper to just choose a 30-MHz GBW op-amp, than to pay a technician to tune circuits in production.


  How do you find such a transfer function?


Many vendors will provide a SPICE model, which will encompass as much as they are willing to share about the transfer function of the amplifier. It may also account for variations of the transfer function due to changing the power supply voltages, input common mode voltage, output load impedance, etc. 

Typically it won't include variations due to temperature changes.

And of course it will only be a "typical" response. As far as I know there's no vendor that provides statistical models of how the performance varies from part to part.