I got an inquiry one night asking if I could make a onesie from a t shirt. I haven't made baby clothes, and at first I was skeptical. I started googling and came across some patterns. Like you find so often, they were of varying quality. One pattern didn't have the crossover shoulders but used the t-shirt neck. One pattern had legs. One pattern you had to import the pictures of the pattern into a word processing program and print them. One didn't have sleeves. Most patterns were too big for what the woman was inquiring about. I ended up using the pattern pieces from HERE and instructions from everywhere.
This was pretty much a quick and easy project. Unless you have a cheap source for t-shirts though, there's no profit. I was looking at Value Village and the men's shirts were around $9.99. How much would someone pay for a onesie?