The city of San Francisco has a lot to offer. The eclectic mix of people from all over the world ensure this city's myriad neighborhoods are packed with interesting things to see, hear, taste and wear. Since this is a menswear column, we'll be sticking with the wearing of things, and since Halloween is right around the corner we'll, of course, focus on what to wear and where to go to get it.
Vintage clothing stores are usually a great place to find a costume for Halloween and San Francisco has a fantastic array of these shops from which to choose. From the Mission to NOPA, from Nob Hill to the Haight, if you can't come up with a costume in San Francisco's vintage clothing stores you're not trying hard enough.
If you are looking for something specific your best bet would be to call around (yelp.com has scads of names, numbers and addresses). If you're feeling adventurous, or if you just cannot decide, an afternoon of bopping around the city should get you into a costume you didn't even know you wanted.
Here's a list of vintage shops grouped together by neighborhood:
Mission
Haight
Castro
Nob Hill
Noe Valley
Hope this was helpful. Good luck!