Snowy Rock - Snowpoint City

In the Sinnoh Region

You can evolve Eevee into Glaceon by leveling it up near a big amount of Ice. 

Sinnoh Region

In the Sinnoh Region, there is a small rock frozen in ice in Snowpoint City. It's the only place cold enough in the whole region to evolve Eevee.

Unova Region

In the Unova Regin, if you want a Glaceon, Twist Mountain is the perfect place for you. Other than the frozen rock is 50-below fairenheight, there's nothing wrong with a rock frozen in ice. Some believe that inside the ice is a Glaceon that has been frozen for thousands of years.
Snowy Rock - Twist Mountain

In the Unova Region