I'm going to tell my wife I'll get sex somewhere else if she won't put out
Why do you want tell your wife that?
She's always got some excuse about being too tired
It can be hard when your wife doesn't want to be with you, but you shouldn't threaten your wife. It won't make her want to be with you, she will just feel forced.
She's been sick a lot lately. If she gets some good rest, she will want it and things will improve.
I am glad you are sticking with her.  If that helps, things might get better, but i would get to the bottom of the reason so that it can help mend the relationship.