Well, I tried to search for a rebuilder and didn't have any luck. However, I did come up with a parts supplier.
http://www.autotran.us/hmparts1.html
My suggestion would be to contact this outfit and ask if they have any customers who are commercial rebuilders, rather than DIY-ers.
Btw, you didn't say what kind of vehicle you have, so I assumed you were asking about the cast-iron 4 speed automatic transmission used in Chevy and GMC trucks in the 50s and early 60s. The only Hydraglide I am aware of is a Harley.
Ray