This is the definitive ingredient in mead making. Without honey, there is no mead.
The properties of honey are near-miraculous. In spite of being 80% sugar, it retards the growth and spread of bacteria through a combination of its physical properties as a colloid, an acidic pH balancing act, and chemical composition. It's effective for treating small wounds to prevent infection - both as a barrier against ambient air and as a natural source of small amounts of hydrogen peroxide. As though that weren't enough, when diluted and combined with a strong colony of yeast, it turns into a tasty alcoholic beverage!
Wildflower honey is produced by bees that gather nectar from multiple unknown or unspecified sources. This is usually the cheapest honey. It's often blended by large packagers to make a consistent product that tastes good in most applications. It's just fine to use this honey in mead, particularly as a base honey for melomels or metheglins. It does not always stand out for traditional meads. That said, small-time beekeepers can yield some of the most interesting wildflower honey due to their local biomes.
Monofloral honey is made predominantly from a single source of nectar. The precise percentage varies. It is produced from hives that have access to only one overwhelming source of nectar for weeks at a time. Some monofloral honeys are very well-regarded for making consistently great mead. Others require a careful hand, if they're suitable at all. All except the biggest crops of monofloral honey are considerably more expensive on average than wildflower honey.
There are many sources on the Internet about the flavors and suitability of monofloral honeys in mead, so there is no need to re-hash those here. Orange blossom honey is a safe bet for nearly any mead application, and it's one of the cheapest, most plentiful honeys available! Clover blossom honey is also plentiful and cheap; it can make a fine base for strongly flavored meads, but is not always suitable for traditional meads. More expensive and interesting monofloral mead honeys include fireweed, tupelo, and meadowfoam. Some honeys, like buckwheat, may not be suitable for beginners. Do some research and sampling before buying and using an unfamiliar honey for the first time.
Woodland honey is available in some places. Caveat emptor - this honey is essentially wildflower (and can be labeled as such), but may be made from larger amounts of sap and aphid honeydew, which can lead to a stronger flavored honey but unwanted piney or bitter flavors in a finished mead.
There is some ambiguity in honey labeling. Read the fine print when selecting honey. Terms like "natural" and "pure" don't necessarily mean anything. "Raw" is defined by the National Honey Board as "honey as it exists in the beehive or as obtained by extraction, settling or straining without adding heat," but this definition does not have the weight of law behind it. Settling or straining removes bee parts, wax, and debris, which is unpalatable to the average consumer.
High heat and extreme filtration can remove, drive off, or break down aromas and flavors. Most supermarket honey has been heated to pasteurization temperatures and filtered finely enough to remove pollen. This gives the honey visual clarity and prevents it from quickly crystallizing on the shelf. Clear honey in liquid form is most appealing to the average consumer. It is not ideal for mead making, but it's also not a deal breaker the packaging company is well-trusted.
If there is a choice, look for honey marked "raw" or some variation of "minimally processed." Avoid any product that has been ultra-pasteurized or ultra-filtered - though such products cannot legally be labeled as "honey" in the US.
Another concern is the geographical source of the honey. There have been recent accusations that honey producers in developing nations mix high fructose corn syrup with their honey in order to bolster volume. This can be very difficult to detect. Look for honey produced locally if possible, or marked with a certification comparable to TrueSource. This might require a little digging, but the results are worth the effort.
There are several honey vendors mentioned repeatedly as solid, dependable sources. The Bee Folks, Dutch Gold Honey, and Miller's Honey
If there is a local farmer's market, check it for local apiaries. Talk to the vendors. If they're beekeepers, they will probably talk happily and endlessly about bees to anyone who will listen. Make friends with them, give them samples, and over time they may be able to provide very unique or rare honey flows like apple blossom or water-white early spring honey.
Most states have a beekeeper's association, and many of those have websites. They will be able to provide contact information for dozens of small local apiarists. These small-time producers may be willing to part with the bulk of their harvest if offered a better price than bulk packagers will provide.
Use the National Honey Board's Honey Locator to locate honey producers by state.
Supermarket Brands / Certified
Whole Foods sometimes has honey available by the pound at a bulk price.
Costco honey is TrueSource Certified.
Dutch Gold Honey and Gunter's Honey are both widely used by meadmakers, trusted, and commonly available online or in supermarkets.
Lighter honey tends to be more delicately flavored; darker, bolder. But this is not always the case.
When deciding whether to use such honey for mead, simply smell and taste it. Getting past the sugar can be difficult, but with experience and practice, one learns to distinguish favorable and unfavorable characteristics when considering a honey for mead. To help get past the sweetness and better reveal underlying flavors, try diluting it with 4 parts water to one part honey.
Ken Schramm's The Compleat Meadmaker thoroughly examines honey.