You can buy the Slider for micro:bit directly from the following retailers.
The MonkMakes Slider for micro:bit allows you to interact with your micro:bit by sliding a control left and right.
The board uses a 10kΩ linear variable resistor (pot) to output a voltage between 0 and 3V that can be measured in your micro:bit programs using one of the micro:bit connections as an analog input.
The kit includes the Slider for micro:bit itself plus a set of 5 alligator clip leads and a cap for the slider. Comprehensive instructions with example projects are provided as a free download.