Costume ideas abound for the emergency services theme party. You can be anything from a police officer to a firefighter to a doctor. The costumes also range from very inexpensive to quite costly, depending on how authentic of a look you desire. Most of these costumes are also comfortable to wear.
You can dress up as anyone who works at a doctor's office or a hospital. The list includes EMT/paramedic, doctor, surgeon, nurse, coroner and forensics examiner. All of these costumes are available at most thrift stores, the most easily accessible accessory being the "scrubs" which doctors and nurses wear. You could squirt a bit of fake blood on yourself to complete the look.
Public Services Ideas
Dressing up as a public service employee such as a firefighter or a police officer is a little bit harder to complete under a budget. This idea requires a bit more of a costume; you might have to buy/rent an actual outfit.
Government officials or employees such as FBI, CIA, or any part of the Armed Forces are a great addition to this theme party. You can go to almost any army surplus store to pick up camouflage coats, trousers and gear. For the CIA and FBI, use a blue track jacket or windbreaker and tape on the letters CIA or FBI in big yellow text on the back of the jacket. Adding some big black sunglasses and a fake moustache also help complete the look of the federal agent.