We do not charge fosters to participate and I would be hesitant to do so because it might be keep people from volunteering to foster. As other's said having a wishlist at various vendors, asking fosters to provide some materials (which is especially easy if they are already pet owners) or having specific fundraising events for foster supplies have been helpful ways we have covered supplies in the past. We have hosted specific fun events at the shelter to gather supplies such as a "kitten shower" day where people can bring a kitten shower present, come play common baby shower games but with cat themes, and play with kittens. These events are also a fun way to recruit new foster parents.
If you have a secure communication channel such as a Facebook or other chat group for your fosters you can try putting a call out a few times a year for specific donations for the program. Our fosters have always been very willing to provide donations if we ask.
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