Where can I buy a copy of TTD?
Sorry, you can't. It's been out of print for years. If you search around the web you can find a place to download it, if the laws of your country and your ethics allow this. We cannot provide a copy for you but neither is it our business where you got yours.
You may be able to find copies of the game second-hand. Owen's Transport Tycoon Station has a page with some suggestions.
When will there be a new version of TTD?
Chris Sawyer's Locomotion was released in 2004 as a "spiritual sequel" to Transport Tycoon. It performed less well than expected, however, and it looks unlikely that there will ever be a sequel.
There are several open source projects that are attempting to make an improved semi-clone of TTD. OpenTTD is undoubtedly the most fully-featured, and continues to implement new features and improvements on a regular basis.