Depends what you want to do with them - if it for morphological examination, you could try the standard palynological preaparation techniques without the HF digestion: dry the material, heat in 10% NaOH, centrifuge, wash in distilled water, centrifuge, wash in glacial acetic acid, centrifuge, react in a mixture of 90% acetic anhydride and 10% concentrated sulphuric acid (careful! highly reactive!), centrifuge, rinse with glacial acetic acid, centrifuge, rinse with distilled water, centrifuge, and you should have very little left other than the phytoliths.

An alternative would be to ash the material in an oven at 500 deg. C, but this might not produce as clean a sample and might damage the phytoliths - but so much simpler it might be worth trying first.

Here is a paper in which we looked at plants to see the phytoliths from them.

Article Phytoliths of Southeastern Vancouver Island, Canada, and the...