Where can I get silver sulfadiazine cream to heal my sunburn?

Expensive overkill. Sunburn is response of the tissues to injury from solar radiation. It is simply inflammation which means increase blood flow to the area to promote healing. Do not use ice as this can injure already injured skin. Cool water soaks can help as can taking anti-inflammation medicines like Motrin or advil (ibuprofen). Avoid any further damage as this can cause scarring . Good skin cream applied daily/gently helps.
Usually not needed. Many doctors prescribe silver sulfadiazine cream, but it isn't necessary unless the burn becomes infected; if that happens, call your doctor. Apply washcloths soaked in cool water as often as possible, and take Ibuprofen or something similar for the pain and inflammation. And please learn the lesson that mr. Sun is trying so hard to teach you: apply a good sunscreen lotion every day.