Horns of Doom
All characters begin with 100gp.
Items of Common rarity can usually be purchased (or acquired by a merchant within a few days) in most towns.
Items of Uncommon rarity my occasionally be purchased in larger towns, but might take some time/favors to acquire.
Rare items are almost never available for purchase.
Most items listed in Pre-essentials books are considered Uncommon, but a list of Common items can be found in this errata update