Kwik Sew and Stretch and Sew both make multi-sized patterns for knit fabrics.
One can sew knits with a straight stitch machine. Use a longer (maybe 8 to an inch) stitch and gently stretch the fabric while sewing it. When it relaxes, there is some stretch in the seam.
In fact, Ann Person came up with that technique.
Sewing with a zigzag stitch is easier, and sewing with a serger is the easiest, but one can make garments with a straight stitch only machine.