Actually Mark, it can be very handy to have fosterers that only have certain kinds of cats, especially pregnant cats. I have fostered for 6 years and rarely had kittens, I only averaged one litter a year, that was more than enough for me, I was happy to let everyone else have the kittens and leave me with the adults and oldies - no one else wanted the oldies, which suited me, as I was the only fosterer without kids - the kittens were better in homes with children so they were more socialised.
HAve you been through the CatChat lists? It might also be worth seeing if you have a branch of Cat ACtion Trust near you.