Every level has a large locked chest. This chests can be opened with the keys dropped by bosses. No other method will work.
The big chest always contains a random piece of equipment, 10 coins and 2 random potions. Equipment obtained in this way is more likely to be an artifact.
There are lots of regular chests scattered around the stages, each containing a random piece of equipment. There is a chance of obtaining two items from a single chest if you possess the Fortune affinity.
Note that some chests are mimics that attack you on sight and drop an item upon death.
a regular chest in the forest
a big chest in the forest