ASI 3019225 Internal Plug for Plastic and Brass Cable Glands, Type TCP12, fits M12 and PG7 Cable Glands, Black (Pack of 100): Amazon.com: Industrial & Scientific
Source PG/M-T type metric cable gland plug on m.alibaba.com
Sealcon CD36AP-BK Nylon PG 36 Dome Solid Insert Cable Gland
Cable Gland Internal Plug For Temporarily Sealing Off-Hanrro
ASI 3019290 Internal Plug for Plastic and Brass Cable Glands, Type TCP45, Used to Seal Open Cable Gland, Order Correct Size, Black (Pack of 50): Amazon.com: Industrial & Scientific