Thermal imaging sensors can easily run into the hundreds if not thousands of dollars for really high end sensors. If you are looking to do some basic thermal imaging on the cheap, you may want to consider the MLX90621 IR sensor made by Melexis. It currently costs around 45 USD from Digikey for a single unit. For that relatively low price you get some pretty decent specs that would be more than good enough for a lot of basic thermal imaging projects.
The IR array is arranged in a 16×4 pixel format so you get a vertical or horizontal strip of temperature data depending on sensor orientation. The sensor itself has a 0.5-512Hz programmable refresh rate and can measure object temperatures from -20 to 300 degrees Celsius. The field of view on this sensor is 30°x120° but it is also available in 60°x15° and 40°x10° versions.
Tindie seller Pesky Products sells a breakout board for this sensor which takes care of the low-dropout voltage regulator and some necessary pull-up resistors. All you have to do is solder the sensor to the board and you are good to go. There is also a Raspberry Pi based thermal imaging project that uses the MLX90621 sensor over on Hackaday.io. The build log contains some handy tips on hooking up the MLX sensor to a Pi.