Tower is the Hobbico outlet and they are manufacturer/distributor of Monokote. If they are out then it is likely that many of the other places that buy from them are out also.

I'm sure it will be back in stock soon. I would imagine Cub yellow is a popular color.

If you have any good hobby shops within driving distance they might have it on hand.

You could also just go with Ultracote as was mentioned.