M-Tron is the best Mellotron emu out there. It's truthfull to all greatness of the mellotron, but also has its downfalls(10 second limit on each note, as had the real intrument).... It contains all of the tape banks ever realeased on the hardware thing and is well worth the 100 bucks.
Zero-G's Nostalgia contains several Mellotron samples as well as orchestron(Mello's cousin) samples. But might not be worth buying for all the other stuff. Although i suspect you'll find old synth paradise in it....
If not i think i have some Kontakt Mellotron sample sets lying around my 4 Terrabytes of samples....(got my hands on a real one and sampled it for myself, and ya this instrument is cursed lol :P) What tapes were you looking for? i might have them who knows?
EDIT: I looked around back in the days when i fell in love with the mellotron, and really there are like 5 plugins for it total. Tapeworm, Nanotron, M-Tron, Zero-G Nostalgia and Sonic Synth(the two latter being huge libraries with some mello in them). The others aren't even worth mentionning..... I use M-Tron and Zero-G's myself....