The IDFPR Division of Banking publishes the largest Illinois bank and the relevant interest rates near the beginning of January every year. You can find this year’s announcement on its website. Note that a different interest rate may apply to each lease; you need to determine the starting date of the lease and use the interest rate from December 31 of the year immediately preceding the inception of the lease. 765 ILCS 715/1.

Also please note that the law you cited, the Security Deposit Interest Act (765 ILCS 715 et seq.), applies only to lessors of residential real property containing twenty-five or more units (not twenty-five or more tenants). It does not apply if the renter is in default. 765 ILCS 715/2. Moreover, the municipality in which property is located may impose its own required interest rate on security deposits, most notably in Chicago and Evanston. See Chicago’s Residential Landlord and Tenant Ordinance, Section 5-12-081.