Your in Novato eh? Hmmm... check out
www.barcelino.com
They have fine suits and the people that work there can definately put together a saucy outfit for you. According to the site, there is one is SF, but I have only been to the one at Hillsdale in San Mateo. In my experience with buying formal wear, I find that you generally get what you pay for. If you want a real classy suit that people can tell is hot, you need to expect to spend at least 300. My gut tells me that for 400-450, you can start getting some REAL nice stuff. If you ever head to England or Italy, you GOTTA buy some suits while your there. They cost WAY less than here and use AWSOME material.
If you can spend 300, go check out Macy's. You can most likely find something nice, but not of the upper echelon status that is given off by a Barcelino suit. Good luck bro and take some pictures from prom !!
EDIT: Just in case you weren't aware, suits are meant to be taken care of and worn numerous times. It's not like you are gonna spend major dough and wear this once. If you get something classy and quality, it is like an investment of sorts.