Where can I find a weatherproof while in use cover long enough to fit a big in line gfci box..?. its for my hot tub...

I doubt you'll find one for something that big, but if it exists a real electrical supply house will either have it or can get it.

Inline GFCI box? You mean on the cord? They are already waterproof.

Inline GFCI box? You mean on the cord? They are already waterproof.IF it is being plugged in outdoors the cover would need to be an in-use type.
I can't think of any that would fit one of those GFCI male cord ends.

You may want to try a greenhouse supplier. ​They have specialty items which must be waterproof because of the nature of the application.