Some mead makers find one yeast that works for them and stick with that. Many enjoy trying different strains to explore unique esters and discover niche applications.
Here are a few nearly-foolproof yeast strains.
Lalvin 71B-1122
Red Star Pasteur Blanc (Previously known as "Pasteur Champagne")
Lalvin EC-1118
Most local homebrew shops carry common wine and beer yeasts. Online shops have a wider selection of yeasts that are not commonly found in homebrew stores. Bread yeast is, of course, available from the grocery store.
Be careful when buying from large retailers like Amazon, especially when purchasing liquid yeast - it may have been stored improperly or sat around long past its best-by date.
Detailed information on yeast types and selection may be found on the Ingredients/Yeast page of this wiki.