5 ILCS 220/3.5
Any expenditure of funds by a public agency organized pursuant to an intergovernmental agreement in accordance with the provisions of this Act and consisting of 5 public agencies or less, except for an intergovernmental risk management association, self-insurance pool or self-administered health and accident cooperative or pool, shall be in accordance with the Illinois Purchasing Act if the State is a party to the agreement, and shall be in accordance with any law or ordinance applicable to the public agency with the largest population which is a party to the agreement if the State is not a party to the agreement. If the State is not a party to the agreement and there is no such applicable law or ordinance, all purchases shall be subject to the provisions of the Governmental Joint Purchasing Act . Such self-insurance or insurance pools may enter into reinsurance agreements for the protection of their members.
5 ILCS 220/3.5