where do u get that extra stowage rack for the rear of the M1 Abrams?

It depends on which style you mean.

2 BDE, 3 ID tanks from APS 5 draw vehicles in OIF 1 had a locally-made in Kuwait square basket added to the rear bustle rack. This is available in the Dragon M1A1HA Abrams 1st Marine Tank Bn, Baghdad 2003 kit (#3533), even though it is incorrect for a USMC tank. It is also in the Dragon M1A1 AIM kit (#3535) w/3 ID options. I believe Edurd makes a PE one as well.

There is also a folding BRE (Bustle Rack Extension) that was initially used by the USMC and is now a standard peice of gear for Army tanks after AIM rebuild as well. It can be found in the Academy M1A1 Abrams Iraq 2003 kit (#13202), along with other USMC specific parts. It is also in the Dragon M1A2 SEP kit (#3536) . Eduard does a PE one as well.