How can I get a DVD of this movie?

If you're in the UK you can usually pick up a copy from HMV or Virgin for a few quid - it's not too hard a film to find and I'd be surprised if they don't have it in stock.

If you're outside the UK you'll have to get it ordered in I think but there's loads on Amazon:

http://www.amazon.co.uk/I-D-Warren-Clarke/dp/B00004W4H5/ref=sr_1_1?ie= UTF8&s=dvd&qid=1209240581&sr=1-1

Hope this helps.