you can buy Fx-991CW calculator. It is popular and more advance, it is upgraded version of FX-991EX. 

https://stickerhk.com/casio-classwiz-fx-991cw-advanced-scientific-calculator

CASIO FX-991EX CLASSWIZ is a high-performance scientific calculator  equipped with a high-resolution LCD screen, which can display more tasks and greatly reduce the degree of scrolling. In addition, it features advanced calculations to quickly solve multi-step problems. Through the online visualization function, users can quickly present various types of data. CASIO FX-991EX CLASSWIZ has two power supply methods to ensure reliable use in any environment. It is suitable for a variety of application scenarios such as calculus, algebra, geometry, etc., and is a very practical calculator