How to obtain raphides from plants?
Searching for raphide purification I came up with a paper where they isolate these crystals from kiwi fruit:

Konno, K., Inoue, T. A., &amp; Nakamura, M. (2014). Synergistic defensive function of raphides and protease through the needle effect. PloS one, 9(3).

Does that help? 

Note, however, that while according to this paper these large crystals can be kept in suspension that isn't the same as a solution ...