Bas,
I don't think anyone makes or sells complete Automatic Transmission Rebuild Kits since the usual procedure is to only replace the parts that are broken, worn, or out of spec's. You can probably buy gasket and seal kits, as well as valve body kits, etc.
I suppose places like AAMCO or other large transmission repair chains may have kits or packs of parts for specific transmissions so that their tech's don't have to order 100 little parts, and simply order a rebuild kit, but these kits are probably not sold to the public.
You would also need a manual on the transmission to know all the spec's, tolerances, torque spec's etc. Some transmissions require special tools and special assembly and sub assembly sequences. If you don't have the tools, manuals and knowledge, you probably should leave the rebuilding of the transmission to a professional.
I recall a few years ago that there is a performance transmission shop in California that rebuilt the Gen1 5 speed automatics and offered standard, heavy duty, and race ready transmission ranging from about $750 to about $1700 (w/o torque converters). That did not include shipping and there was a $350 core charge on your old tranny that you would get back when they got your old tranny. They also sold some heavy duty parts and valvebody kits for the 5R55E tranny.
The company is called California Performance Transmissions. They are still in business and have been even doing work for "OverHaulin" the TV show. I looked them up about 6-8 years ago, so I'm sure their prices are much higher now.
You might want to check with IPT Performance Transmissions. The build Ford racing transmission and sell a heavy duty/racing version of the 5R55E for around $2800, and they claim they will ship to anywhere in the world? If it cost more to ship your old tranny back, it might be better to just pay the core charge and keep your old tranny and sell if for scrap or parts over there?
...Rich