Have you tried writing to GWR?
Hello There,
I Cant Provide You With 5 Go Great Western But I Can Provide With Plenty Of Other Poster Including SWR Posters, South West Trains, First Great Western, GWR Posters Ranging from 2010-2018 also i can provide coach nameboards to bristol temple meads, lonon paddington, york, london kings cross
Kind Regards Peter White