I would bet that you can find used calculus textbooks quite easily.

I did chemical engineering long ago (how long? Computerized process control was just being introduced into the curriculum, and we learned to program in Fortran). I did several calculus courses, but it was always just a matter of memorizing techniques.

Much later, I came across Calculus Made Easy on Project Gutenberg, a textbook from 1914 that actually helped me to understand why calculus works, instead of just treating it like magic.

https://www.gutenberg.org/files/33283/33283-pdf.pdf

Calculus hasn't changed a whole lot. There are probably better books for learning than I used in the 1970s, but I have to believe that you can find pretty decent older calculus texts for not a lot.

All textbooks are free if you know where to look