Also, any letter before the 74LS is usually a company initials. Don't worry about as long as it says 74LS-what ever number you need, its the right chip.

Make sure it is a "DIP" (Or Dual In-line Package) - 2 rows of pins. Not one row ("SIP" - Single In-Line Package) or SMT/SMD (Surface Mount Technology/Device). On some online places will have this information and others not. Just make sure it is a DIP (usually by a picture on the site).