Do you have access to an ordinary sewing machine? And want to do a little work on it yourself?
I found that button thread and amazon vinyl make decent seat covers and doesn’t take much time or talent.
Hardest part is attaching it to the seat pan if you don’t have the tools or hardware for that.
My kids seat was an old T-shirt, 1” foam, vinyl. Stitched through for the ribbed look and stapled onto a 2x4 with more cushion padding. Took all of 1 hour, learn as you go.
You’ll have way more vinyl and foam than you need and it will be inexpensive