2 yrs in and the toilet is flexing and making noise every time you sit on it. Not sure how to fix this or if I’m going to have to remove the entire toilet. help would be great!

I just installed my memoirs white toilet and the seat and lid are black. My plumber installed it thinking I wanted mismatch toilet and seat. How can I order a white toilet seat? The seat and lid is square. I can’t seem to find this toilet seat. How can I order the correct white seat?