I don't think you can buy just the seal/gasket without getting a whole new cap, but if you are delicate you can use a dull edged flat blade screwdriver and remove the gasket from the new cap and slip it over your original cap. This way you don't lose the tether.
You can get a Toyota OEM new or used one on AMAZON right now for under $20.
Amazon.com: Genuine Toyota (77300-33070) Fuel Tank Cap Assembly: Automotive
I don't think you need a new gas cap if it's not broken. I just replaced a gasket (77316-33042) on the cap which was much cheaper.