I want to replace vinil or plastic beading on my kitchen bar stools. Do you know where I can buy it please

If this is at the upholstery seams... you won't... its sewn into the material used to pad the seats. You would have to rip out the stitching and resew everything, you likely won't do this on a home sewing machine... if its not there could you please explain better or provide a picture of where exactly it is.

Find a reupholstery shop. They usually will sell you some. Also Google "Hidem seat welting. " very commonly used in boat seating.

Do you mean piping, material wrapped around thin rope and sewn into the edge?