It doesn't need to be thinset. There are lots of different companies that make tile mastics in small containers. And don't tell anybody but I've used liquid nails and it's worked fine for a small patch. I probably wouldn't use it in an area that's going to be wet a lot like inside a shower just because I don't know what happens to liquid nails after months of being damp.
The problem with something like liquid nails is it gets all over, can't be removed easily and leaves ugly dark brown spots if it oozes out at all between tiles while your setting them.