From an earlier thread - Mike Anderson tried some buttons from Siriusconinc for his 350 and reported:
Just wanted to report my results with the chrome buttons which I purchased from Siriusconinc. com
The buttons arrived promptly and were just right for the seats. Their buttons are well made and will hold up great. You need to use a washer on the backside because the shoulder above the split is a little more than the thickness of the seatpan and cover. Call me at (317)729-2418 or
e-mail at fleagle14@embarqmail.com if I can answer any questions for you.