This is probably the most prevalent transmitter sold in America. It is called the Genie Intellicode Transmitter. Or GIT-1 for Genie Intellicode Transmitter with 1 button. These rarely need new batteries and rarely ever go bad. They have been in production since 1993 and probably half the houses in America have one. They use rolling code technology so there are literally billions of codes.
These take "N" type batteries which can be bought at radio shack or take the whole transmitter into sun city hardware and they'll replace the batteries for you making sure the polarity and such is correct. Sun City hardware, 28250 Bradley Rd 951-679-3656, is usually less expensive than Radio Shack and their battery selection is fresher since they are the source for batteries in the area.
We always keep plenty in stock and along with us at all times.