Why don't you buy the yardage, wash it up and shrink it (it's a baby lovey, it's going to get wash a million times, right!?). Then tear it at the length you want it. Flannel shrinks a TON; if they rip it for you before washing, it's not going to be 42 x 42 once you pull it out of the dryer the first time anyway.
My MIL used to get diaper flannel and make swaddling cloths out of them. She turned them and and embroidered the edge. They were rarely square, and I can promise, nobody cared. That nice soft diaper flannel is dreamy.