Horsetail or that new ocean sourced stuff in that article are best sources. However only young shoots of Horsetail before the Silica begins to crystallize.
Nettle has some and good quality Silica in small amounts but its main job is to protect the Silica from being flushed out of the Kidneys.
You may be able to find an article on Nettle use during WW2 in Germany, and how some of the millions of Germans starved by the Allied Forces survived. Finally eating the last vegetation in ditches which was nettle..
At least one European Homeopath still makes it in pill form to 'strengthen the constitution'..