Cyclon III starter Kit, flash device

I don't know about the CIII kit, but Terasic usually does provide a flash controller in Verilog. It is present in the Control Panel source code.

However Terasic demonstrations are usually too bad for learning purposes, they are barely commented if at all. Furthermore, the control panel doesn't attempt to be very efficient. E.g, most versions of the Terasic control panel doesn't let you erase individual sectors.

It might be much better to use a CFI controller. It shouldn't be too difficult to find one, and I think that you get a clear source code (but of course, not commented) one when instantiating a CFI core in SOPC builder.