Mutable instruments Clouds is the most popular module for granular effects. Mutable allows for clone builds of a lot of their modules. There are micro versions (smaller footprint) versions of Clouds. At this time, my favorite is "Monsoon". Monsoon is relatively smaller, less expensive ($250US) and allows for independent modulation of attributes that can't be simultaneously modulated.
There are other sampling/granular modules out there as well. But in a Eurorack system, the oscillator/voice is just the beginning.
In all honesty, you'd be better off spending your money on a copy of Ableton Live augmented with the Max4Live device "Granulator II." If you have an existing modular system, adding a granular/sampling module isn't so much of a stretch. But starting a brand new system around granular synthesis is going to be a very expensive one-trick pony.