Hi
I foster for the RSPCA too, I have fostered for two different branches before. The first one I was given more reign to advertise etc, as the bunny I was fostering was a really long stay bunny (the branch don't usually use fosterers, but I had adopted from them before and they knew me). I advertised her and she went to a member on here
I provided accommodation and paid for all food, hay etc and drove her back to the branch if necessary. I could have claimed for hay, food and petrol but felt wrong to do so. I would have claimed for any vet bills due, but luckily she never needed the vet whilst with me. She was with me for nearly 1.5 years and went back to branch when I went away (and moved house!)
Earlier this year I started fostering for a different branch of the RSPCA, this time I provided the accommodation (although they would have bought it for me if I had needed it). I have had 4 foster bunnies so far, two singles and currently have a pair. Again, I pay for food, petrol etc (although sometimes buns are dropped at mine or bunny runned over) but would claim for any vet bills. The bunnies I have looked after were for shorter periods of time, perhaps 3 months or so. The two I've got now are older and both black so I fear I may have them for some time
The buns are advertised and re-homed by the co-ordinator, I don't advertise them for her (she does bonding for the RSPCA too).