Where can I get a copy of Mavericks after Yosimite has been released?
Apple sells redeemable codes for all OS older than current release up to and including Lion from http://store.apple.com

Since the SKU change from time to time, you may need to talk or chat with a sales specialist online to get the correct item into your cart.

I've never had problems just reinstalling the original OS that came with your Mac and then using the App Store to upgrade to a now down-level version of OS X. Also, one you've "purchased" the free or paid upgrades, you can always re-download them from older clients in the purchased section.

Basically, the old OS even going back to 10.6 and earlier are all available for purchase and re-download by either contacting AppleCare or the Apple online / retail stores in your country.