Try searching for the brand spasilk, they carry a preemie size for 3 to 5 lbs :-) I used to use that brand in my printing business I had before my spinal cord injury. Gerber also has a preemie size onsie, but there not as soft and are a bit longer. I had started my business because of preemie babies. I had a grand daughter that was born at 25 weeks and weighed less than 2 lbs, and was in the hospital for over 3 months :-) she's amazing now and perfect, but she was the one to inspire the start of my business when I went to find cute preemie stuff, and realized there wasn't much to choose from :-) the spasilk preemie onsies are sooo nice though, and they even have very cute contrast stitching on all the edges. Pink for girls and blue for boys. I use to buy them from Burlington coat factory as it was the only place I could find that brand, but it's worth googling as they are sized so nicely.
I found them here right now on amazon, even some with cute print :-)
Amazon.com: Spasilk 100% Cotton Assorted Short...