Well perhaps this isn't the place for this (Maybe Photrio forums is more like it), but yes, buying expired film on eBay will do it. To me, buying a mid-speed film that expired in the 1st half of the 90s might be the best balance between seeing a noticeable difference and not too dangerous. I've shot and processed 126 film that expired in the 80s and there was definitely a lot of speed loss. Very thin negatives. The older the film, the slower you need to rate the film. This is the wild card method, as you don't know how the film was stored all these years and the results are completely unpredictable. High speed film goes off faster of course and is more risky. However, grain structure of vintage films is much different than those of today and can't be replicated. Just make sure that it's still a C41 film and not made for some earlier process.
You could go thew harris Savides route, take a contemporary film and bake it at a low temperature, but you'd have to test it a bit to find the working ISO speed and baking time/temperature that gives you the look you want.
Another technique might be to slightly underexpose a fast film while shooting through a colored filter. When printed back to normal, it will induce color crossover. For example, If you want red shadows and cyan highlights, shoot through a somewhat strong cyan filter. If the shadows are thin, they will exhibit red when you correct back halfway.
Alternatively, Adox makes a film called Color Implosion that is designed to look "old" or "vintage" with, according to them, color layers that are "collapsed." Do a search on this film. It may be too "70s" for you, or it may be perfect.