I've made my own. They are symmetical front to back as well as left to right.
Making the legs is just like making the sleeves of a sweater, except you have to scale up a bit. I do mine in the round on a set of four double pointed needles. I increase by one stitch on every eighth needle until I am at the required circumference for the thigh. If you put a mark at the end of one of your needles, and turn it over each time you use it, it will help you keep count - decrease when you come to the marked end.
Once you get to the required circumference, work straight until you are up to inside leg length. Cast off ten stitches.
Make another leg, just like the first. (Actually, I make my legs in parallel, doing a bit of one and then a bit of the other, checking for symmetry from time to time. You do need to have plenty of double-pointed needles for this though - I make mine from dowel.)
Then you need a rectangle for the crotch - do a provisional cast on and stocking stitch about ten stitches wide and about ten rows long.
There is then a moment of mayhem when you join the legs to the crotch patch. Knit round the first leg to the point wheer you casted off ten. Then knit across the top of the crotch patch, and round the second leg and across the other end of the crotch patch. Then you are back where you started.
After the mayhem, I settle down to using a set of five long double-pointed needles. You can use a flexible circular needle if you like. Put markers at the front and back of the thighs.
You may find now that you have more stitches than you need for the waist. For about ten rows, you K2tog (through back of loops if necessary) above where the crotch patch joins the legs. After that, you decrease where you put the markers, four stitches every few rounds, to get down to the right size. When you get to the waist, you do about three inches of K2P2 rib and then do a loose cast off. You fold the K2P2 rib in half and sew it down, leaving holes for the drawstring. The drawstring is about 4 feet of i-chord - I think 5 stitches wide is about right.
Sew up the slits between the crotch patch and the legs.
You can do this baggy in double knitting wool, or tight using wool-acrylic-nylon blend. I have tried tight using pure acrylic, but it wears out too quickly. The drawstring is best done in wool-acrylic-nylon blend - strong and stretchy. Do the plain stocking stitch stuff on 4 mm needles, and the rib on 3.25 mm needles.