One suggestion might be to contact the shelter and see if they can help you with low cost spay/neuter. A lot of shelters offer reduced costs especially since youaren't asking them to take the cats.
It sounds like you have wonderful setup for the kitties and it's a wonderful thing you are doing. I just hope you can get all the females fixed as soon as possible so you don't have to worry about more. I know it's hard and expensive but hopefully the shelter can help you out. If they won't then I woudl call around to some of your local vets and explain the situation to them and see if they will discount for you.
Good luck and keep us posted.