Pretty much the smallest quantity of mulch you can buy is the standard 2 cubic foot bag. The good news is, it's cheap (about 3.75 right now for hardwood mulch). The bad news is, you'll have a very hard time finding it in the city anywhere but Home Depot or Lowe's. (Don't buy the dyed garbage, go all the way to the end of the row and get HARDWOOD MULCH, no dye, no bark)

However, you can get FREE mulch in any quantity you want up to 30 gallons at the Organics Recycling Center in Fairmount Park, if you are a Philly resident. However, you will need to drive there, bring your own container, and pay attention to their not-totally-convenient hours: https://www.phila.gov/services/trees-parks-the-environment/get-organic-materials/