A magnifying glass, handy if you want to examine an item with small markings. A list of what you already have, if you're looking to add to a set. Comfortable walking shoes. An umbrella or rain coat if rain is forecast. A large canvas bag or backpack for lugging your newfound treasures.
Some folks use a collapsible luggage tote.
In addition, some people prefer to bring along their own drinking water and food, although many large flea markets have food and drink available. If it's going to be a hot day, try placing your water in the freezer a day ahead of time. Wrap the bottle in a towel to slow the thawing and to absorb the condensation as it melts. Finally, make sure you have room in your vehicle for any large item you may buy. Throw some old blankets and rope in your vehicle too.