Buy Ableton Live between July 1st 2009 and August 31st 2009, authorise the product, and you will automatically get a serial number for Sampler. It's already part of Live 8, so you don't need to download Sampler separately.
This offer is valid for purchases of Ableton Live 8. It is also valid for upgrades to Ableton Live 8 from versions of Ableton Live Lite and Ableton Live LE. It is not valid for educational and not-for-resale products, or for upgrades from older full versions of Ableton Live. Purchases or upgrades must be made between July 1st and August 31st 2009.