I think any Toyota dealer should be able to order you a new interior heater/radiator (not the big one under the car) based on your chassis number.
But maybe it is not necessary.
Basically it can be because of two issues: Air in the system (so not proper heat dissipation) - or a dirty clogged interior radiator.
I had the same issue, it was not so powerful. I flushed the coolant systems two times to get brown stuff out.
But it was only untill I put the gardenhose into the hose going to the interior heater and put it full open (lot of water and pressure) that major brown stuff was coming out.
Now it is delivering really hot air again.
So disconnect the hoses going to the interior heater/radiator (not the big one under the car), and put a garden hose at full power into it. And let it run for a while until it stops giving brown stuff, and only clear water is coming out.
Then top up with engine coolant again, and let it run for a while with the cap of, so you can add water when air is coming out.
Don't forget to enable heating inside the car, otherwise it can't get through. Also check this valve, as it should go open when enabling heating from inside.
You can check this by laying under the front of the car, behind the front bumper you should see two hoses going up (coming from engine compartiment)