Old Model-M keyboards are still sold at http://www.clickykeyboards.com/ - or ebay.
http://store.apple.com/us/product/MB110LL/A?fnode=MTY1NDA1Mg...
http://matias.ca/products/index.php
also see (some long threads)
My current favorite is the Logitech Wave... before that, I used a Das Keyboard -- which was fantastic, but the space bar became detached after about a year and a half.
i dropped it like maybe 100 times for the past 3 years or so, it breaks but none of the button breaks. it's easy to reattach loose button to keyboard (yes, even the space-bar)
it's always wise to invest on keyboard, you are practically married to it ... just don't buy new (mine's used, bought like Sing$10)
if you're opposed to buying used, think about it ... you test the used kb, find it's usable, nothing breaks ... what that tells you? ... the used kb has stood the test of time and abuse for years ... then marry her
does the trick, comfortable as well.