Whether you need a lab coat for a school project or for a Halloween costume, the best route to go is to buy a cheap, used lab coat. Lab coats are relatively easy to find at a variety of locations; however, you'll 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.
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 online. 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 online 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.
Make sure you know what size lab coat you need and ask on the phone if that size is available before heading out to buy the coat.