I want something conservative. I like how they look obviously, but the practical benefits are more important to me. Where is the cheapest place to get one in the US?
Remember that if you're buying something cheaper, it may not last as long, have a poorer fit, be made of a less comfortable fabric, etc., Also could you be more specific as to what kind of onesie you're looking for? Diaper shirt onesie, long john pajama onesie, long john pajama footed onesie, etc.
I like the Abena Bodystockings. They're plain white, like underwear, and fit well. They're cheaper than ones with fancy ABDL designs, and they're really well-made. I've had mine for years.
Little For Big onesies are cute and ok for the price. Gets the job done. But if you have big arms, I don't suggest them. They ride up in the armpits for me and it gets pretty uncomfortable.
Also if you're female you might find them a little uncomfortable without a bra. At least that's how I feel.