Look up contact information for a local locksmith company or hardware store. Most chain hardware and home improvement stores offer key cutting as a service. Look in the yellow pages or do an Internet search for local locksmiths or hardware stores.
Call the stores ahead of time and ask if they can make a key for your particular needs as some stores may not have blanks for every type of key. Some keys, such as one for a safe, may require documentation to show you legally own it, so ask if any papers are necessary to have the key made.
Bring the key and any necessary documents to the chosen store. The entire key making process should take no more than a few minutes.