BEFORE YOU BUY!
TRY TO BORROW FIRST!!!
Go to your school library or local library and try to find a physical or digital copy and just borrow that. Textbooks are expensive for most people and are usually a waste of money unless you can afford to just straight up buy one without any issues.
Cambridge Mathematics Mathematics Extension 1 Year 11 and Year 12
New Senior Mathematics Mathematics Extension 1 Year 11 and Year 12
Terry Lee New Essential Mathematics and Terry Lee New Fundamental Mathematics
https://www.advancedmathematics.com.au/products.htm
Maths in Focus Mathematics Extension 1 Year 11 and Year 12 (Personally don't like the questions in this one but use it if you have to)