Contact the company that sells the furniture.

https://www.bedbathandbeyond.com/Home-Garden/Satin-Stool-26-set-of-2/39463870/product.html?option=80659148 They are sold at a few different places. Google Lens will provide you with several retailers and price ranges. The chairs posted from BBB are sold in sets of two.

Here's another idea: If you have any of the fabric left from making the seat cushions try placing it on the seat of the new chair. Can the fabrics work together? If no, then forget the next suggestion. 😊 If yes, consider painting the wooden parts of the new chair to match the legs of the side stools - it looks like they might be a light gray - and then using that fabric make a THIN (3/4"-1" foam) round cushion the same size as the chair seat to sit on top of it. To stay in place on the padded seat, you can hide a slightly smaller piece of rubberized gripper cloth that goes under rugs to prevent sliding (see below) between the new cushion and the original chair seat. OR perhaps make a thin square or rectangular cushion to rest against the back of the new chair which might make the chair more comfortable as well. The idea is to tie the new chair in with the cute side seats using the same leg color and cushion fabric. You have a beautiful kitchen!!!