Whether you need a lab coat for a school project or for a Halloween costume, the best way to go is to buy a cheap used lab coat. They are relatively easy to find at a variety of locations. You just need some time and patience to track one down.
Know what size you need and decide if this is this a one time purchase or something that you would like to buy in bulk.
Video of the Day
Start your search by looking in the phone book under uniform supply stores. Call the store and find out if they sell used lab coats.
Check out a medical college supply store. These stores often purchase lab coats in bulk and you may be able to buy one at a deeply discounted price.
Search on-line. If you are unable to find used lab coats at local stores you will need to get on your computer and check eBay or other on-line auction sites. You won't be able to pick up a coat the same day as you buy it but you will be able to find a wide selection of cheap, used lab coats to suit your needs.