I know that a TI-83 emulator out there, but you need to get the rom using a Graph-Link cable and a computer. It's free!
TI 89 Emulator
I don't know how well it will work because I have never used it.
There is also Math Expander, a powerful graphing calculator, but it is very hard to learn and even harder to actually use. It's worth a try anyway. Also free!
Math Expander
There is also a third commercial program which is the most powerful out of all of these, and it is reasonably easy to use. You can probably forget about using your fingers to run it--you'll need a stylus. It costs $30.
Mobile Space Time
Hope this helps