Generic Chilly bottle from Wilco’s, available in a wide range of colours, and cost five quid. Others are available for three to four times that, I doubt very much that they work any better. I’ve got two, one for tea, green in my case, and one for water.
As above, after boiling the kettle, I fill the bottle, then pour the water into a Pyrex jug with a teabag, let the bag brew for a couple of minutes then pour back into the bottle; if I make one at 7am, I can finish it off at my break at 5pm, it’s still warm enough to be quite drinkable.
Brilliant things, for about the price of a pint, incredible value.
If the inside gets stained by tannin from the tea, I put a denture cleaning tablet in with some warm water and leave it filled to the top, cleans the inside perfectly.