Be safe just get one from the dealer.  You could probably find one on eBay,
but the dealer is usually faster, then having to wait.

The first time I replaced my mast, I ended taking out the whole assembly to
remove the broken part.  The second time around, I just unscrewed the cap
removed the mast, and turn on the radio.  The broken nylon center will come
right out.  Next insert the new mast and with help from another person have
them turn on and off the radio.  You could do it by yourself, there is
usually enough time.

I bought one at Autozone for $20 (1996 Camry) and another from an
on-line Toyota dealer for my 2001 4Runner for about $30.

Often you can change the mast without removing the whole antenna. If all
of the old plastic linear gear comes out when you pull out the mast then
you can just stick the new gear in there and the gear on the motor shaft
will grab it (pay attention to the orientation of the old one as it
comes out). If there's pieces of gear left in the motor housing then you
have to take it apart. Getting to it was kind of a pain on my 4Runner.

It's a very common replacement part for after-market parts stores to
carry. Pep Boys had a place for it on their shelf, but they were out of
stock.