Where can I purchase/download UCI Pro Tour/pro cycling race footage? I love watching old races - they're especially great for long sessions on the trainer. However, I can find only a few stages/races on Youtube, and most of the videos are simply highlights. Where might I be able to purchase/download pro race footage (licitly)?
There are sites such as The Sufferfest who do videos designed for trainers. I suspect there will be a bunch of these sites, you've probably found some already.

Also, some of the turbo manufacturers (I'm thinking particularly of Tacx since I have personal experience here) release DVDs which can be used in conjunction with your trainer. Tacx do two types - one is just a DVD that you watch while you pedal (and so is not restricted to Tacx trainers), the other is integrates with the Tacx software to give you a virtual reality experience (i.e. the speed at which you pedal governs the speed at which the video plays). But both are filmed in the first person, as if you're one of the riders, rather than being a commentary. See the Tacx site for more information - http://www.tacx.com/en/products/films. For the most part though these are courses rather than races, although there are some races and these are probably close to what you're looking for. But they work out very expensive.

Lastly I know the Tour de France release videos every year, but as you say these are highlight videos rather than full-length races.

But I don't know of anyone offering exactly what you're asking for I'm afraid. Certainly the European races, the spring classics etc, I'm pretty sure that videos don't exist. I mean, you can find loads of clips on YouTube but these are bits and bobs recorded off tv, I don't think you can get an "official" 4 or 5 hour dvd. I stand to be corrected...