Also forgot to mention this one:
Flight1.com - Great Aviation Products
This and the Mindstar offering are 'gauge' addons that you can retrofit into your desired aircraft. Though retrofitting them requires some editing of the panel files for the aircraft in question. I use FS Panel Studio to aid in this.
Alternatively you can shop around for FS 2004 ddon aircraft that come with default glass cockpit displays, though there are few of them. Two I can think of are:
Quest Kodiak Webpage
and from my pals at Eaglesoft:
General Aviation
Be mindful that the MFD functions within FS 2004 addons will not have the fidelity of the real Garmin G1000 or Avidynes due to limitations in the Flight Simulator engine. For greater levels of G1000 MFD fidelity you'll need the standalone Garmin Trainer software (unsure of current price and suppliers).
I do believe the Avidyne trainer is free thoughh and is available at:
Avidyne EX500 and EX5000 Training Software