Hey there Mr. Cunningham
There are 3 decent places i know of to download new things for it and to be honest, those 3 are so big that you will not need to use any others!
I can reccomend aircraft to you as have tried out the majority of them and there are some seriously good ones out there - some tested by real pilots.
the 3 places i use are...
<< Simviation (or Simaviation) .com >>
<< Flightsim.com >>
<< Avsim.net >>
I personally usually use Simviation for a few reasons - you dont need to register unlike the other two, the site is basic and easy to navigate and there is a list of the days new files. The aircraft are also catagorised into what type of plane (civil, prop, miltary etc) and also which version of Flight Sim they belong to. Pictures are provided by the descriptions too and you do not need to open a new page to download a file like you do in Avsim.
As for your Drag CoEfficients, everything that affects the handling of an aircraft is in its "CFG File" - this is a file that you can open in notepad and is specific to the type of plane its pinned to. If you want to be clever you can edit things to change the way a plane flies but i havent done so. Looks easy though.
It looks like this - this is a small part of the CFG for the AN124:
;Moments of Inertia
empty_weight_pitch_MOI = 90000.000
empty_weight_roll_MOI = 70000.000
empty_weight_yaw_MOI = 130000.000
empty_weight_coupled_MOI= 0.000
CG_forward_limit= 0.000
CG_aft_limit= 1.000
[flight_tuning]
cruise_lift_scalar =1.000
parasite_drag_scalar =1.000
induced_drag_scalar =1.000
elevator_effectiveness =1.000
aileron_effectiveness =1.000
rudder_effectiveness =1.000
pitch_stability =1.000
roll_stability =1.000
yaw_stability =1.000
elevator_trim_effectiveness =1.000
aileron_trim_effectiveness =1.000
rudder_trim_effectiveness =1.000
Just things like that - pretty simple i guess.
Anyway, you did a good thing getting that game

it will give you many hours of fun. Im sure you will love it.
Anything else you need to know just say...this is something i do actually know a fair bit about lol...
regards
Chris