Do a WWW search for heavy duty 42RLE's.... there's a few US based companies that do them.... but you'll have to do the costing carefully as they are damm expensive and all teh shipping and core charges and and add up.
Rebuild kits for the 42RLE are also not overly expensive from the states.... the local rebuilders IMHO screw you over on parts, so supplying them yourself might be an option.
As for experienced auto rebuilders..... they are all "experts" when it comes to taking your money
but modern autos are electronically complex and simpler mechanically that older hydraulically controlled ones so a rebuild should be within the capabilities of any professional who has a clean workshop, the right tools and can follow directions.
In reality its not likely to be cheap so perhaps see if you can source a second hand unit before diving in.