Gift to a supportive professor?
It's nice that you want to do this for your professor. Thinking over what I would like (and have received in the past), the following comes to mind:


a note (email, letter, whatever you prefer) saying essentially a longer version of what you said above. A heartfelt note of this kind is the best kind of thanks. 
if you'd like to get something more tangible for the professor, a little token that they can place on their desk (like what Pete L. Clark mentions) is always nice. It doesn't even have to cost lots of money. I have students who made paper models for me (I teach geometry) and I have them all over my desk :).