Shiny Ferroseed is in Pokémon GO now thanks to a Mexico exclusive 7-Eleven weekend. If you currently live in Mexico, to participate in the event you'll need to register your event ticket in the 7-Eleven app before Oct. 21 and spend at least $150 dollars at 7-Eleven locations.
Doing this will grant you access to timed special research, increased spawns of Bulbasaur, Squirtle, Sableye, Charizard, Patrat, Minccino, Mawile, and gives you access to shiny Ferroseed. For those of us outside of Mexico, it is unclear whether or not we'll be able to start encountering Shiny Ferroseed on Oct 21, however, once the information is cleared up you'll know how to catch one.
Shiny Ferroseed Pokémon GO
Shiny Ferroseed can be differentiated from regular Ferroseeds by its turquoise spikes and rings around its eyes. Shiny Ferrothorn is a little bit more obvious with its pink vines and gold body. To catch a shiny you could always wait for an event like the current fashion week event or the upcoming Halloween Event, during which certain types of Pokémon will spawn more frequently. If you don't feel like waiting for an event featuring Ferroseed to come around there are other things you can do, like focusing on trying to catch Pokémon when it's snowing to take advantage of increased steel type spawns.