Can't speak for other rescues Helly, but this time of year is horrific. Many fosterers or cat workers are away on holiday themselves, which leaves the rest of the organisation doing double workload. With us, all new fosterers need to be visited and 'inducted' into procedures. Sort of like a little day school.
Then they need to be equipped with stuff... food, hygiene equipment, some carriers and basic meds. If they are to have a pen in their garden, that pen needs to be purchased and built and hooked to electric. The list goes on...
So while on the whole rescues are desperate for fosterers, they do take time and finances to set up. Sometimes, as it is with us from time to time, we struggle to find the money to finance a new fosterer. Sometimes we can't spare the time to properly introduce one.
I am sure someone will get back to you and scoop you up when the dust settles.