a simple op-amp circuit should do the trick, when you want constant current across a resistor that can change due to temperature, it is sometimes best not to use other resistors because in application (vs theory) it can cause voltage changes. An op-amp will hold the current no matter what kind of load you hook up to the output of it, so it would seem that it would be the best bet for you. You can change the feedback & input resistors to change the current and it will not affect the load