There are many kinds of honey, based on which flowers the bees collected the nectar from. Bees aren’t loyal to any particular flower, so any characterization of honey as being from a particular source (for example, “blackberry honey”) can vary from absolutely true to a rough generality, depending on what flowers the bees can find and how interesting they find them. Honeys range in taste and color from the light clover through alfalfa to stronger tasting (and darker) such as buckwheat. There are many unusual honeys to be found where there are unusual local flowers. Which honey you will use depends both on which you like the taste of, and what type of mead you are trying to make. Stronger flavors go well in metheglins and heavier or sweet meads, while the milder honeys make a good base for melomels or dry traditional meads. Realize that a honey with an interesting-but-unusual taste can produce an overpowering character in mead.
You can buy honey in bulk from roadside stands or health food stores. You may be lucky enough to live near an apiary and be able to buy right from the beekeeper. Look in the phone book for honey, health food, or beekeepers. Sometimes, exterminators will remove hives, give the bees to beekeepers, and sell the honey. University agriculture departments occasionally sell honey. Be inventive. If all else fails, you may have to buy it from the grocery store.
The honey will be either raw or processed in some way. Raw honey has bits of wax, bee parts, dust, pollen, microorganisms, and the like in it. You have the most control in how you process raw honey, but you also have the most to do. Honey may be filtered, or blended, or even heat-pasteurized to make it clearer and less likely to crystallize. The more processed it is, the milder it is likely to be and the less character it will give to your mead. The processing also dissipates some of the honey’s aroma. Commercial, “grocery store” honey, crystal-clear and pale, is the most processed and is usually not a good choice for mead making.
Crystallized honey is normally acceptable for mead. In fact, it has two points in its favor: First, it generally indicates less processing, since one of the reasons for processing honey is to keep it from crystallizing. Second, it may be cheaper because it’s less appealing to the average consumer. (One point against crystallized honey is that if the sugar is drawn out into large crystals, the liquid surrounding them can be low enough in sugar content to allow some fermentation from wild yeast.) To re-liquefy crystallized honey so you can pour it, just heat it gently.