Where can I find a turbine shaft for a bv39-0031 turbo?

Or a chra

The internet?
http://www.xsboostturbochargers.com...acement-chra-bv39-vw-1-9-tdi-brm-engine-code/

I believe that's for the bew engine not the brm or it's possibly for the European model since the golf and beetle didn't get a brm engine

Deaership or just have it rebuilt. Cores or (CHRA)'s are mostly all rebuilt in china with the finest chinesium ESPECIALLY if you buy from ebay or any source that has not been around very long or does NOT go to the track and drag race the car they built AKA Darkside etc....

Have a professional rebuild it or buy new from a REPUTABLE source
that link is sketchy AF bro. No seriously scroll down, it was made in 2017 by some cheesy web service but they have been in business for 12 years but is NOT accredited by the BBB. They have some REALLY BAD reviews on the customer service and backing up their product or services. Plenty of good reviews too and FWIW seems to be a OK shop but i would do ALL your business with these guys OVER a REAL Credit card so you can have the backing of the card if something goes bad and dispute the charges. Food for thought.
Spend some time calling around and searching for turbo builders etc...