I have a 35yo baby rocker that needed a new runner. I got that made and was told the wood is poplar and the rocker is currently just natural with a clear finish. Now I need to stain or paint it to match a toy box, crib, grow to furniture that are all espresso. Is is going to be easier to spray paint the rocker or stain it? Can I even find espresso stain? Im thinking with the new runner and the older chair paint might be best choice for continuity but not sure.

You can definitely get espresso stain. HD and Lowes will both have it.

If the piece has a clear finish on it, I wouldn't think it'll stain very well until you get that off. Stain needs to have access to the grain and be able to soak in.

i am concerned that it is tight wood AND has been sealed.

You can't stain the chair until the sealer has been removed. Either get to work sanding or paint.