The cheapest place and they ship anywhere in the world, plus they accept creditcards & paypal:
http://www.futurlec.comIt only took 11 days to arrive an oscillator to Chile!
Could you post a photo of the oscillator you are going to change, not all oscillators are the same: some are 14 pin dil others less, some 5volt, others 3.3. Depending on accelerators design and processor you should require a different speed. So please give a little information on what you intend to do.