They come in two lengths, standard or karbine. Can be bought direct from hull cartridge or somewhere like chambers gun spares.
It has a gauge on the end, with a small hole and plastic plug just behind the guage that is to accept the QF probe.
Looking at about £100 or near as.
Theoben TTR1 .177, Steven B Blue/Grey Laminate Stock, Nutshot Mod, Falcon Menace 10x42 B20 Ret