At the Mefferts site http://mefferts.com
they sell megaminxs brand new for $26. I think this is actually only 6 colours, but from the same site you can order a set of 12-coloured tiles for $12.
For $36 its a lot cheaper than you will probably find on ebay etc. However if you are looking for an old original one or something you might have to pay a lot more.
Hope this helps! And good luck.