Beer cans can be found in a variety of places including old houses, barns, dumps or camping sites.  Older cans can be found in pits that were used to dispose of trash during hunting or fishing trips in the 30s, 40s, and 50s.  A good metal detector is essential in finding these buried treasures. 

Cans found in these places are called dumpers and are typically in less than pristine condition.  Sometimes, however, you find rare finds or decent cans depending on the location and age of cans.  Typically, older cans (prior to 1940) tend to be in better condition since the paint and metal of these cans were heavier.  Cans found deeper in a dry area are often in better condition.  Sometimes, you may find beer cans inside other cans (e.g., coffee cans) that are in good condition.  I recently dug a grade 2 bud OI can that was found inside another can. 

Dumpers can be cleaned using powdered Citric Acid or Oxalic Acid.  I typically use Citric Acid since it is cheaper and easy to obtain.  Oxalic acid tends to be stronger, but may harm cans if left in the bath too long.  To clean cans, first wash them using soap and water.  Get all of the dirt off of them.  Then soak them in an acid bath for a specified time (always use protective gloves and do this in a well ventilated area).  The length of the acid bath varies depending on the strength of the acid, age of the can, and amount of rust.  After the acid bath, soak the cans in soap and warm water.  Gently brush the rust off with a soft toothbrush.  Dry them in the sun or with a fan.