I use this in my tommee tippee one - https://amzn.eu/d/3bUP2CE
I also have the electric tommee tippee steriliser. You can use white vinegar (fill up the hole with it) then put the lid on and leave it an hour or so. Then pour it out, wipe the shelf and inside of the lid down, then put it on a ‘cycle’ without anything in so it doesn’t smell. X
I have the tommee tippee steam one and I use citric acid. You can buy it online or places like the range do a tub for like £1.49 half a scoop on the baseplate with the recommended water and just run a normal steriliser cycle.