Andustrial 3-in Square Strut Bracket in the Strut Accessories department at Lowes.com
Amazon.com: UNISTRUT Genuine P2950-EG 4 Wheel Trolley Assembly for use with P1000, P1001, P5000, P5001, P5500, P5501 and All 1-5/8" or Taller Strut Channel : Industrial & Scientific