My son discovered a hack on YouTube that seems to work - quite a few YouTubers have tried it with success. If you have an Eevee and enough candy to evolve it, you can choose which incarnation you get by renaming it. Sparky gets you a Jolteon, Rainer a Vaporeon and Pyro a Flareon. I believe these are the names of the original trainers for those Pokemon. To do it, rename your Eevee, then close and re-open the app to ensure the name change has been registered by the game servers, then evolve the Eevee. There is speculation it won't work for the same kind of evolution twice in a row, or even twice at all. Here's a full explanation: https://youtu.be/-XnsN0MZPzg