You can get vista beta 2 from MSDN or by participating to the program named ‘Windows Vista Customer Preview (CPP)’.

 From MSDN a DVD version of 3200 MB is available with :

For driver developers, a development kit is also available (2392 MB) : Windows Vista Beta 2 Windows Driver Kit (WDK)

Ms link for beta 2 development : http://msdn.microsoft.com/windowsvista/downloads/products/getthebeta/