I went through this too a while back.. Good luck finding it for a reasonable price. I think the whole set is $150+ on Amazon, though if you haunt Ebay you may get lucky.
I do have some tips though.. One is that the your PC serials work fine on a Mac, so if you already have the Windows version of game and can find someone to lend you their Mac install disks you'll be just fine.
If you don't have your PC serials you can also buy a new copy of NWN Diamond for PC and use those for a Mac install. But don't use serials from a used game, a lot of them are bad.
Another tip is that there is something called "openknights" that will install the PC expansions if you have a mac disk of the original game and the PC expansions. It's out of date now so it's tricky to use (I think you have to install the 1.69 update halfway through, but I could be wrong, it's been a while.) I got it to work after futzing with it for a few hours but that was a couple years back and I don't remember now what I did.
If all else fails there's always Boot Camp and installing XP. That's pretty much a last resort though.
