I doubt it is going to sell out any more than any previous edition of Mac OS, since the $29 is the full price and does not reflect any temporary discount (and thus there is no need to run off and get it straight away). And since Apple is dropping the price so much, I am sure they have market research indicating how much more CDs will be sold because of it and manufacture accordingly.
With that said, I am sure you can do it through Amazon, perhaps even Apple will have pre-orders closer to launch date?