Alan Whitmore gave permission to repost his comments from the Nc-rockets listserv:
Because of the increasing problems with customer service at Walston Retrieval Systems, there has been a lot of interest in other suppliers of small radio-tracker transmitters that can be used with our current Walston receiver, which is working fine. I have done some research and come up with a few companies and web-sites that you may all be interested in looking at. [Dang! I just ended another sentence with a preposition. The grammar police are definitely coming after me some day.]
www.marshallradio.com Look at the Scout VHF transmitter. You can request any frequency in the 216, 217, 218, and 219 MHz range (remember that our club-owned receiver covers the 216.000 to 216.160 MHz range, so ask for something in there, unless you want to buy your own receiver.). Jim Livingston has one of the Scouts, and it works extremely well.
www.radiotracking.com/xlf.html Look under the tab labeled "aero models". this is the LL Electronics tracker that the flyers from VA and MDRA have been using for years, and it is also excellent quality and works well in rocketry apps. The website does not mention transmitter frequencies, so be sure to be specific if you do business with them. This is a much stronger device than the Walston, because transmitter, batteries, and antenna base coupler are all contained in an aluminum tube.
www.merlin-systems.com click on the "Rocketry" tab. 216.000 to 216.160 transmitters are available upon request. I've never actually seen any of their products in action, but the product looks good.
www.telenax.com/en/ This is a British company that specializes in wildlife tracking equipment of ALL sizes. They don't actually have a dedicated 'Rocketry' section of their website, but that doesn't mean that the product wouldn't work extremely well for our purposes. I was especially fascinated with the implantable trackers that could be placed in bats for tracking their movements. About the size of 2 match heads. You could put one of these in the nose cone of an Estes Mosquito, and not upset the flight characteristics! Cool!