If you don't have a Grove shield for a flame sensor, you can use any flame sensor module, for instance, the flame sensor module from SainSmart. This sensor module enables the Arduino and Raspberry Pi to be attached to the board. This sensor module is cheap, and you can find it at https://www.sainsmart.com/ir-infrared-flame-detection-sensor-module-detect-fire-flame-sensor-for-arduino.html. This is how it looks:

Basically, a flame sensor is cheap and you can find it on Aliexpress, Banggood, or another online store. A flame sensor usually has three or four pins: GND, VCC, A0, and D0. Some flame sensors have the A0 pin or D0 pin or both.