Vintage Options

Why buy a vintage bicycle?
If you are on a tight budget, buying a vintage bicycle is often a better idea than buying a low-cost modern bicycle: Chances are, the vintage bike will be better in quality and will last longer. Just be aware that you will need to make some updates to a vintage bike that will require additional spending - so factor that into the purchasing price. Please read this for additional details of the benefits of vintage vs modern bicycles on a budget.

What kind of vintage bicycle to get?
Assuming that you are female and wear "office appropriate" clothing at least some of the time, there are two basic types of vintage bicycles that are suitable for everyday transportation. I have written detailed posts about each:
. the Vintage 3-Speed
. the Vintage Mixte

How old should the bicycle be?
I would say that ideally, the age of the bicycle should be late-1950s to mid-1980s. Any older than that, and it is likely to require much more than a basic restoration, plus you may have difficulty finding suitable components. Any newer than that, and the quality may be approaching modern standards.

Where to buy a vintage bicycle?
The major sources of vintage bicycles are Craigslist (or similar local online resource) and used bicycle shops. If at all possible, buy locally - try the bike in person and inspect its condition before buying.

How to Refurbish a Vintage Bicycle?
There are different methods, depending on your goals and, of course, the type of bicycle you have. A good place to start, is by reading the following:
Refurbishing Vintage Bicycles: Different Perspectives
Lovely Bicycles on a Budget: Vintage vs Modern
and then, once you decide on a course of action, visit your local bike shop or the Classic&Vintage subforum of bikeforums.

Are there resources for further advice?
Yes. Best thing to do, is to visit the Classic&Vintage subforum of bikeforums. You will get all the advice you need there, and more.

Good luck with your search! A properly updated vintage bicycle is a joy to ride. For additional tips, read this post. And visit The Boston Retro Wheelmen -  an informal society for the appreciation and preservation of vintage bicycles.