Some people make equivalents of the plastic parts out of wood etc to get themselves going.
For actual plastic ones you pretty much need to get them from someone who already has a printer up and running. I bought a makerbot at the beginning of this year, and am currently printing out the last of the pieces to make myself a mendel. Some of the large parts have been pretty tricky to print on the makerbot in ABS. Once that's up and running I'm happy to print out a couple of sets of parts. In terms of price, the "reprap way" is that once you have built yourself a printer you should print out two sets of parts for fellow reprapians for the cost of plastic, which I'm *guessing* would be about $30 (assuming it would take about 1 roll of Vik's PLA to get out a set of parts).
Alternatively Vik is working with Tim at mindkits.co.nz to do complete mendel kits, so you could email them if that is an option.
Cheers,
Len.