The CIMA wants to make sure that the calculators you bring along is not programmable. In the past, there has been instances where candidates (in CIMA or other professoinal exams) used such calculators and gained an unfair advantage in the exam.
Even for the approved calculators, the exam administrator may reset them prior to the exam.
You can use the computer-based calculator on screen. While some Pearson VUE centres may have calculators available, they have no obligation to lend to you, and the calculators there are very basic.
You can practice using the computer-based calculator by downloading the practice exam tutorial from the Pearson VUE site.
Among the CIMA approved calculators, I recommend Texas Instrument – TI BA II Plus. This calculator is approved in different accounting and finance professional exams (including the CFA exam). It is also affordable and easy to use.
I am the author of How to Pass The CPA Exam (published by Wiley) and the publisher of this and several accounting professional exam prep sites.