You can season your own wood. Hardwoods (Ash, Oak, Beech, etc) take over two longer to fully dry out, which is more than softwoods (pine, larch). Hardwoods do burn hotter and longer than softwoods, giving more fuel efficiency and brighter flames. Of all hardwoods are the general consensus of opinion for the best wood to burn is Ash, sometimes known as the King of Firewood. Green wood is best cut and split before being left in a dry. Your store should be a sheltered area with plenty of ventilation to season. Keep the wood off the ground, do not simply cover the wood with a tarpaulin unless you are willing to remove it frequently. Seasoned wood will absorb moisture to replace what it has lost if it is left exposed to rain or in contact with the ground during a frost. Wet wood will rot faster, even properly stored and seasoned wood may not last more than five or six years of storage.
When seasoned properly the wood will be darker at the ends with visible cracks or the grain starting to split, it will also make a distinctly hollow noise when struck, as opposed to the more solid thump of green wood. Most importantly having had most of its water removed, seasoned wood weighs considerably less than fresh wood. Moisture meters are available that can determine the moisture content of your wood, most sources agree that 20% moisture is the absolute maximum for burning.
Wood can be purchased already seasoned or kiln dried. When you buy it check that it is seasoned properly. You should ensure that your wood comes from a properly managed source and this can be checked quickly and simply through the Forestry Stewardship Council.