Alt 1: Get the original
As only 10k units were ever created, the original 909s have become rare and expensive. As of writing: only one item is listed on the second-hand marketplace Reverb. Listed at $7000 :o. Check out current offerings and cry.
Alt 2: Get an emulator
Hardware clones: either created by Roland or other companies.
Software emulators:
Alt 3: Grab the samples
Raw samples: wave-files.
Samples played in sample software: Kontakt, Ableton.