Shops that have experience rebuilding NV-5600 transmissions and are able to obtain repair parts for them are pretty rare.
I would have a local shop perform the labor to remove the transmission, install a complete new clutch assembly, and reinstall the transmission but strap the transmission on a pallet and ship it LTL truck freight to Blumenthals in OKC or Standard in Fort Worth. I have had personal experience with Standard and recommend them but others have had a similar experience at Blumenthals. Both are TDR advertisers, well known to members, and completely professional operations.
This is a situation where it will be much wiser in the long term to let the pros do it.