You can only deduct the cost of the uniform if it is not suitable for everyday use. The cost is an out-of-pocket expense in giving services to the organization. Enter the cost in TurboTax as a money donation.

If you don't have any other use for this uniform, list it as the donation of an "Item" (clothes, furniture, cars, etc.) in the Donations to Charity in 2020 interview under Personal Income > Deductions and Credits > Charitable Donation..  Use the name & address of the charity you volunteered for.

You will have to itemize deductions in order to claim any charitable donation.