Element for 6 litre thermo pot herms

Hi guys. I have a 6 litre thermo pot from Bergland that I'm planning on converting to a little herms pot.
I've seen people use elements that are fitted in the bottom section of these pots bit I'm insure where people get these specific elements from? What size do they have to be? I would ideally like to fit one in the bottom and have the cable feeding through to the outer bottom rim so I can fit a gland to it and wire on a plug to end of the lead.
I've seen people use elements that are fitted in the bottom section of these pots bit I'm insure where people get these specific elements from? What size do they have to be? I would ideally like to fit one in the bottom and have the cable feeding through to the outer bottom rim so I can fit a gland to it and wire on a plug to end of the lead.