Stanley no longer sells replacement gaskets for their older thermoses, but a large industrial O-ring will suffice. Find a hardware store with a large selection of O-rings and you’re good to go.
In fact you can often find old Stanleys very cheaply at thrift stores or garage sales simply because they no longer have a good seal. – DB in Oregon
Just a quick note with some info that might help. One liter liter/quart Lexan Nalgene bottles (an presumably other brands, though I haven’t tried them) make great Hot Water Bottles when filled with boiling water and covered with an old boot sock. Just make sure the lid is screwed on firmly and then tighten it just a bit more once the lid is good and hot.
The Hot Water Bottle I have the most experience with is nothing more than a 2 liter soda bottle that I filled 75-80% full of water and then squeezed all of the air out before closing the bottle. This allows it to expand as it is heated in the microwave without rupturing. If it gets firm when it is hot let a little more water out until there is no pressure on the bottle once it is good and hot. Kept in an old boot sock and heated in the microwave each night (Experiment with how long it will take with your oven) it will keep you warm for more hours than most of us get to spend in bed in a couple of nights.
Many mornings we awoke with ice on the inside of the windows and our breath readily visible while being nice and warm all night with this simple combo while living in an uninsulated cabin a few years ago. The 2 liter soda bottle was heated nightly for something like two winters or a bit more before it failed so they can be pretty durable. – S.D. in West Virginia