Here’s a secret the stone industry doesn’t entirely want to share with you: you don’t need a fist-sized stone in order to evolve. All you would need is enough energy within a single stone to trigger evolution. Granted, there’s only a finite number of times you can split an evolution stone before it becomes ineffective (typically, that limit is when the sample is roughly the size of a key stone), but in the case of evolution stones found in the wild, you may be able to get one or two cuts from it without losing its effectiveness.
That having been said, take your shiny stone to a reputable jeweler and ask them to cut it into two. They should know what to do from there. It may not be the least expensive solution, but it will certainly save you more than a few headaches.
Besides, finding a shiny stone on the market isn’t easy. You may be able to find them in the Sinnoh Underground (which is a tourist destination, not a euphemism for their black market) or online, but I would be wary about online retailers, especially on PokéBay. Fake stones aren’t that uncommon. If you choose this option, always buy your evolution stones from a reputable seller, such as EvolutionMarketplace.com.