Where can I purchase the bumper bar?
Hey, Ginger,
Spare parts are very hard to come by, so your best bet would be THE seller of your pushchair. They should simply order it for you from an official distributor for your country or directly the manufacturer.
Next in line, you can also try to contact the manufacturer's (Peg Pérego's) customer service for your country.
And, lastly, you can browse classifieds or eBay – there can be someone selling just the part you need (in this case, a belly bar) or the whole (e.g., damaged) pushchair for parts.
That is pretty much all you can do. I wish you the best of luck! And don't forget to leave a review of your Switch Four HERE to help other parents by a minute of your time ^_^
Your -very berry- Eli.