Those “T-Bone” shifters are probably some propriety part that Roadmaster specified for your bike. You might not be able to find a replacement anywhere. Fortunately, those types of shifters are usually compatible with generic Shimano-style indexed and friction shifters. So you can match the number of gears of your bike to a basic Shimano-compatible thumb shifter which should work. For example, a generic 7-speed shifter that is compatible with Shimano index shifting should work for your 7-speed bike. Just match the number of speeds. Most generic brands are Shimano-compatible, along with Shimano’s basic index shifters such as Tourney. Don’t get grip shifters or hub shifters such as Nexus or Alfine because they won’t work for your bike.