I think the teapot with 'built in' tea cozy cover is a postmodern-mid century (1930s-50s) era design. Could be art-deco, not sure. They're a sleek conical shape; the cover slides over the top when the lid is removed.
You see them on Ebay (US/Canada/UK) every so often. Keep a sharp eye out, and you should be able to find one in good condition.
Else, you can purchase a regular teapot and find a tea cozy design to suit. They work just as well, like sweaters for a teapot to keep the contents hot.