- 音声合成素材集 - Mac音ナナ［Macne nana] - Mac ソフトのことなら act2.com -
I say use program very loosely because Macne Nana needs the program Reason to run. If you already own reason then this product is probably worth the fairly low price of 30 dollars (seems to also include Macne Petit).
Macne Petit, requires only GarageBand though, since its a AIFF collection, it will likely work with other audio editors such as Audacity. Since modern Macs ship with iLife this requirement isn't high and the price is reasonable (under 10 dollars) but the amount of effort to mix different audio files for a human-like sound is difficult. I might pick it up for 10 dollars but I can't envision seriously using it.
This ain't the mac answer to Miku but the price is right.