I've had this problem before and purchased 2. Then placed them up front.
In fact, I kind of have it again. So your thread is relevant to my needs.
I understand the wisdom is to use the new tires in the back, with the expectation that it prevents oversteer. However in the rain, the front tires do move water away from the rears at highway speeds. Overall, I've found that I can go faster before hydroplaning with fresh tires up front.
The suggestion of a used tire is also interesting. Might be difficult to achieve, but I'm not against it.