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Call Best Buy back, and tell them to check that you made a purchase, at such-and-such a date and time (refer to your receipt, best, ***** ***** your memory of the purchase, less good), of such-and-such an amount, of an iMac, and you left the package behind. Ask them to look for a package empty of an iMac, that would correspond. Possibly in the trash ? or presumed to-be-discarded.
IF the CS4 you bought was Adobe [Photoshop/etc.], then contacting Adobe would be also do-able. But first refer to your iMac... is it already installed, or is there a 'stub' or shortcut for installing it? if so, you may not need to *get* the package or discs, just the registration/activation of the Adobe package in its installed location on the computer.
If/when you contact Adobe, refer to that number on the package. It may be all you have, for relations with them, except perhaps for your purchase slip if that slip mentions/shows an Adobe software purchase on it.
Lastly... this is kind of a last resort.... you could RETURN your iMac, and your Adobe purchase, etc., to Best Buy. If they'll refund for it, you go to another branch, or another store altogether, and buy what you need before going overseas... and be more careful this time, to get everything you buy, in-hand.
This is not likely to work at all, if you don't have *something* in the way of Adobe to give back to them. That's why I mention it as a last resort... for, to be quite honest, this sounds like a badly mismanaged situation. I'm sorry to have heard about it, and I appreciate that it's a pain as it is.