FM is the most obvious way to go I think, so any FM synth will help. 2 sine operators will generally be enough to cover a lot of ground mallet-wise (be it music box, xylophone, marimba or whatever). Just try modulating one sine by another, try playing with the envelopes (typically I make a shorter, pluckier envelope for the modulator than the carrier) and the pitch (usually I’ll pitch the modulator some octaves higher than the carrier when I want to achieve some mallet sound). Try other waveforms than sines (especially on the modulator) to give it another flavour.
As for existing instruments, boscomac has some very nice free reaktor ensembles, and there’s some mallets in there : lamellae is a physical modeling marimba/bell/vibraphone/whatever, twinkle is a physical modeling xylophone, and boxymuse is a sampled (I guess) music box.
Orange Tree has a very nice sounding sampled Kalimba : Grand Kalimba.
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