It's a really kind gesture and a very worthwile thing to do.
I have 3 people who foster for me, sometimes it's only a week or two and sometimes it's for many months, all depends.
I provide the hay and dried food and usually give £5 per week towards veg etc for two piggies or if I've time take some food round to them.
It really helps when I am full here or someone needs extra attention.
I would get in touch with any local rescues but make sure you know from the start what is expected of you, both in terms of money and time. It's always better to get the detail sorted out early on as who does what. For instance I don't expect my fosterers to find homes for them , do home visits or make trips to the vets - I like to do all those things myself.
There is also the emotional side of things to consider like when they leave you..............! Or you could be like my friends who were just hopeless and kept the first two they fostered and then couldn't take anymore LOL !