The principle of a Belt drive consist of these three components:
Timing Belt (Which delivers the Mechanical power)
Idler Pulley (a pulley with a bearing to guide the belt frictionless)
Timing Pulley (To Drive the Belt with a Motor, or to extract power from it)
In general, you start with selecting a specific Belt. EG. 2GT belt with 6mm With. From there on, you can search for compatible componenets, eg 2GT Idler Pulley or 2GT Timing pulley