Lowes, Home depot, Menards or other hardware stores with plumbing supplies.
Fernco makes a 3in to 3in sewer pipe adapter that is black rubber and includes clamps It fits over the 3.5 LS1 Maf and compresses easily to fit a 3 inch Maf pipe. I got mine at Menards for under $4.00. Bob Bailey is one of the people that suggested them.