You can purchase this from a lot of vendors like BavAuto, RMSeuro, Pelican Parts. I buy by the case and used a location that sells Pentosin ATF1 for around 169 bucks shipped on sale.
European Motor Oil - Pentosin Transmission Fluids

Even Advanced Autoparts and O'Rielly's say they sell if for 17 a L.
Esso LT 71141 and Pentosin ATF1 are totally interchangeable and I just changed a friends X-3 filter and refilled the pan with this. Transmission will live a long time doing this.
By the way ZF calls this fluid Lifeguard5.
There you go...