I recently needed to do this and spent a bit of time scratching my head trying to work it out! So here is a step by guide in how to find the url of any tweet.
1. Sign on to Twitter
2. Post a Tweet
3. Click Profile
4. Locate your Tweet
5. Click on the time stamp link of the tweet. (i.e. about 2 hours ago)
6. This will open up the individual tweet.
7. Copy that URL from the Address bar.
And that is that, easy when you know how eh?