I know a guy who works for Flir, and in a conversation with him, I was using the term "IR" and he corrected me, saying that thermal imaging was in a lower frequency range, and much harder to work with. He's a tech guy, not a marketing guy, so I tend to believe him.
But, regarding less expensive alternatives, if you have a compatible smartphone, there are thermal imaging options that leverage the phone and it's circuitry and operating system. So you're only buying part of a thermal Imager, but you're getting full benefit.
Here's the Flir version, some of the others may be even better...
I noticed that this version uses two cameras and it merges a visible image with a thermal image. It could be because this product's thermal image is very low resolution. on the other hand, this seems like it could be a good idea, because I was having a bit of difficulty, looking at the Flir 7 images and pinpointing exactly where a particular, small hot spot was.
BTW, other thermal imager/phone options were mentioned earlier in this thread.
-AC