no clue on making but go here http://cosyndry.com/

When you say "onesie," are you referring to a snap crotched t-shirt or a pair of footed pajamas?

snap crotch cause, footy pajamas are footy pajamas

I once bought a t-shirt that was a size too big and got strip snaps from a fabric store. I used a sewing machine to sew the strip of snaps in place on the front of the hem of the front of the shirt and the back of the hem of the back of the shirt and that worked pretty good until I could buy a real pair.

http://www.wikihow.com/Make-a-Baby-.....from-a-T-Shirt

This is for a baby sized one, but the same steps can probably be taken to make an adult size onesie.

I don't know about the men's section but I saw some at Target in the women's clothes Kyle lol.

Im kinda a bulky and tall guy dought ill fit though.