Do you mean you want to buy tops separately from bottoms, or it's okay to buy a set as long as it's two pieces rather then a nightgown?

I have some pjs and summer nightgowns from LL Bean. They're pretty basic, but they've held up well. I think they might run a bit large -- I usually wear a medium in their sweaters and shirts but a small generally fits me better for pajamas. They have great customer service so you can call them and ask for sizing advice if you need it. And they have free shipping and a great return policy.