Buy some cheap Lucky B/W film. It will expire soon and the look is actually already as if the film would be expired ;D But seriously ... it's the better option for getting an a little bit more vintage or artistic look.
Regarding real expired film, I wouldn't use it. It's a risky thing. In many cases, you just get nothing later and not what you want. Considering, that maybe important pictures get lost because of that and you needed to pay for the development as well, I wouldn't do that.
There is no real market for outdated film, nor any available in any great amount. In the good old days there was a camera shop in Liverpool, F.E.Jones*, which sold a seemingly never ending supply of outdated and government surplus film and paper. It was wonderful for experimenting with and most of it gave results as good as the pukka stuff. I remember some particularly good 10x8 glossy paper.
* Bizarrely there were two camera shops in Liverpool called F.E.Jones. They were not connected in any way other than name. The shop in the city itself was the better for the outdated and surplus supplies.
Similar experience with outdated film,mostly C41 stock, as Dave has had. I have a box full of frozen film in the freezer that I sometimes use. It was frozen before the expiry date and 10 years on seems to produce good results. It is C41 colour stock. However, you never know how expired film has been stored so you could get some nasty surprises.
Bob, I think you have hit the nail on the head there. If it is known to have been stored well it will go on well past its date. If it hasn't it might not even reach its date. The potential problem is that the one 'perfect photo' gets taken on the only bit of film which has gone past its best.
The basic question for me is also, WHY people want to use expired film. Is it the hope for getting it cheaper, the longing for using maybe discontinued films still in a few years ... or is it a special look ?
If it is just about getting it cheaper, shortly outdated film might be no problem but 10 years or so ? Like Bob said, there can be some nasty surprises and I already had them as well. Storing film for a later use ? Why not ? If it is stored properly. But some people want expired film for achieving a special look, which is quite popular especially among toy- and lomo-fans. Here it is just about being lucky or unlucky, I think. There are many interesting results, posted by lucky users, but it's a risky thing. I would say, that it is just luck, getting really what you want.
I bought some very old plus-x pan film once and got some excellent results. I don't think that outdated B&W is as much a problem as outdated color. Some people collect old film. Some for the graphics on the packaging, some for the brand name. I collect old Ansco items (Ansco was based here in Binghamton), so I DO buy any old Ansco items I can find, film included. They made some very good films. I also use outdated films on cameras I've bought just to test the cameras' functions. There are a lot of different reasons for buying and using, or just collecting, outdated films.