I do not know how many rescue organization are in your area or how far you would be willing to allow the kittens to travel, but I think your BEST bet is to find an organization to take them in. It can be done. The organization I foster for just accepted a 3 legged kitty as one of our own. We will make sure it get the necessary surgery and then will have a place to stay until he can be adopted by the right family.
Write a detailed letter and include the fact that you would be willing to continue to foster (if you would be) but that you cannot handle the full brunt of the medical costs (maybe you could add that you could HELP with the costs, if you can). Then send it EVERYWHERE you can find.
The 3 legged kitty I spoke of was found through a petfinder classified ad. He was in our area and one of the volunteers just happened to run across a cry for help written by the person who has him. This volunteer asked a number of the other volunteers and they agreed that we could try to help. So stories like this can have a happpy ending.