http://www.rpmtransmissions.com/hummerpricing.html
Here is one place that I found. It show a list of improvements made to hold more power. If you talk to a place about buying parts, be sure to tell them of your plans (Nitrous, Supercharger, turbo, etc) cause some parts are subject to change.
They appear to use Precision Industries Torque Converters which are reputable in the DMAX crowd