You can deduct expenses to prepare for a foster child if you meet the requirements below.
Per IRS Publication 526:
You may be able to deduct as a charitable contribution some of the costs of being a foster parent (foster care provider) if you have no profit motive in providing the foster care and are not, in fact, making a profit. A qualified organization must select the individuals you take into your home for foster care. You can deduct expenses, including mileage, that meet both of the following requirements:
1. They are unreimbursed out of pocket expenses to feed, clothe, and care for the foster child.
2. They are incurred primarily to benefit the qualified organization.
IRS Pub 526
Unreimbursed expenses that you can't deduct as charitable contributions may be considered support provided by you in determining whether you can claim the foster child as a dependent.