Councils in cities of the first class are hereby empowered to authorize the director of supplies and purchases in cities of the first class to purchase or procure, without public advertisement for bids, and without entering into written contracts, materials or commodities to the value of five hundred dollars ($500.00): Provided, That written bids therefor be first invited from at least three persons, firms, or corporations regularly engaged in the business of manufacturing, dealing in, furnishing, or supplying the same.
53 P.S. § 13372