I've been on the hunt for infant-sized embroidery blanks. Have you tried to find plain onesies and t-shirts in stores? Most have already been embellished. I ordered from 2 sites last week and the service with both was excellent. I ordered Monday and had both orders by Friday.
I ordered shirts and diaper covers from Dharma Trading Co. The downside for me is that Dharma only sells white items. They sell dyes, but that sounds like a disaster waiting to happen to me. The shirts are Rabbit Skins and are very soft. Dharma's prices are very reasonable and they offer a discount if you order at least 12 items (any combination of 12).
I ordered rompers and dresses with panties from Embellishments. Again, great service and nice quality. A little pricier, but many colors to choose from.
Do you have a favorite source for embroidery blanks?