No need for a 'special adaptor PCB': you simply need some FPC breakout board (i.e. Adafruit 50 pin 0.5mm pitch FPC Adapter; get two because you will need one for the data signals and another oner for the backlight, a 40-pin FFC - get if from here i.e. 40-pin FPC Extension Board + 200mm Cable, the FFC connectors i.e. 40 Position FFC, FPC Connector Contacts, Bottom 0.020" (0.50mm) Surface Mount, Right Angle and 10 Position FFC, FPC Connector Contacts, Bottom 0.020" (0.50mm) Surface Mount, Right Angle for the BL).
Wire it to the RPI, configure DPI and start using it.
Source: https://www.raspberrypi.org/forums/viewtopic.php?p=1010869#p1010869
Adafruit 50 pin 0.5mm pitch FPC Adapter @ EXPTEC (Sowas, aber schon bestückt mit Stecker?)
http://www.exp-tech.de/adafruit-50-pin-0-5mm-pitch-fpc-adapter
Adafruit 40-pin TFT Friend - FPC Breakout with LED Backlight Driver @ EXPTEC (Sold out)
http://www.exp-tech.de/adafruit-40-pin-tft-friend-fpc-breakout-with-led-backlight-driver
40-polig, FFC, FPC-Flachbandkabel 200 mm x 0,5 mm, 5 Stück ; Modell: AWM 20624 80 C 60 V VW-1; Rastermaß: 0,5 mm (Amazon)
https://www.amazon.de/Nach-vorn-40-polig-FPC-Flachbandkabel-St%C3%BCck/dp/B00W8WAOKO/ref=sr_1_fkmr2_4?ie=UTF8&qid=1486217174&sr=8-4-fkmr2&keywords=40+Position+FFC%2C+FPC+Connector
50 pin 0.5mm Pitch FPC Flex Cable - 250mm / 10„ long @ EXPTEC (Ausverkauft)
http://www.exp-tech.de/50-pin-0-5mm-pitch-fpc-flex-cable-250mm-10-long
50 pin 0.5mm pitch top-contact FPC SMT Connector @ EXPTECH
http://www.exp-tech.de/50-pin-0-5mm-pitch-top-contact-fpc-smt-connector
Experimentierplatine Epoxyd (L x B) 65.10 mm x 26.14 mm 35 µm Rastermaß 2.54 mm Conrad Components FPC40P Inhalt 1 St.
https://www.conrad.at/de/experimentierplatine-epoxyd-l-x-b-6510-mm-x-2614-mm-35-m-rastermass-254-mm-conrad-components-fpc40p-inhalt-1-st-1274827.html