I used this: https://www.amazon.com/gp/product/B00ZZ93346/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
But if you have higher maths to do as well you might want to consider one of the nice TIs.
Yeah I agree but do not get es or es plus but get ex instead.
Basically this one is good https://www.amazon.com/Casio-FX-991EX-Engineering-Scientific-Calculator/dp/B00ZZ93346?th=1
As a working engineer, I disagree. In my opinion the TI-36X Pro is slightly harder to use and less suited for engineering that the fx-991EX (which also allows you to easily grab and use previous expressions and answers). Not that the TI is in any way bad, but I think the Casio is somewhat better.
$17.28 for the Casio
https://www.amazon.com/dp/B00ZZ93346/
18.57 for the TI
https://www.amazon.com/dp/B004NBZB2Y/
Another good choice:
Sharp EL-W516TBSL at $20.49
https://www.amazon.com/dp/B06XS6V17P
None of these are bad choices, but I think the Casio is the best.
The Casio FX-991EX is the best scientific calculator you can buy. It's also about $20.