How can I thank a senior faculty who's written me a good reference which helped me land a desirable job?
A thank you card is definitely a good idea and I cannot see that it would be considered inappropriate. If you are from abroad, a little something typical of your country of origin may also be appropriate. Like a small box of traditional sweets, a little traditional doll, a fan, or something like that. I have had this kind of little things given to me by students. 

Also, the university will probably have a policy on gifts which you may be able to locate on their web site. The general idea is that it should not be something worth a significant amount of money.

Rather than asking a classmate, I would ask a junior faculty for what goes as the norm. A classmate may be as inexperienced in this as you are, because the rules in academia can be significantly different from other parts of society.