There are a lot of places where you can get nice calculators for pretty cheap. Here are a few:
TI-83 Graphing Calculator: Download a graphing calculator onto your computer!
a Free TI-82, TI-83, TI-83+, TI-85, AND TI-86 emulator for your Android phone.
I understand this is an awesome app, but it takes skills to set it up.
a Free TI-73, TI-81, TI-82, TI-83, TI-83+, TI-84+, TI-85, and TI-86 emulator for your Android phone.
It’s pretty easy set-up. Also available for Mac (although it’s a tougher set-up).
iPhone / iPad:
I’ve yet to find a more beautiful and intuitive graphing app. Everything on it just works and it’s easy to use. The only drawback is that there aren’t a lot of regular scientific calculator functions.
The price is right (free) and it does all the usual scientific / graphing calculator features. Because it’s free, though, there are some advertisements on it (you can upgrade to ad-free for a dollar!).
Although there are a lot of simpler apps for the iPad that do one or two things, this app does it all. But you pay a price for it too: $30