Hey Olivia,
I don't think it would be impossible to replace the keys yes the immobilizer may be an issue but maybe have a look into Techstream,
It's pretty much the diagnostic software that is used by Toyota on early 2000-2012 models. It can be used to search for trouble codes but also has a programming software for things like new keys and immobilizers. I think that's probably your best bet.
All you need is a laptop and a mini VCI cable which can be found on ebay if you just punch in Toyota Techstream. As far as installing the software it can be a bit tricky, there is a good write up on the IH8MUD forum page if you just search up Techstream Installation
I can't say if it will work for sure since I have never had to replace the keys etc but it may be a start.
Hope this helps