Can I buy a dual sim tray for my US iPhone 11 Pro?

No. With an iPhone 11 that was not purchased in China, Hong Kong or Macao, if you want dual SIM you use one physical SIM and an eSIM. See: Using Dual SIM with an eSIM - Apple Support

You can always buy an aftermarket one, but it will only work with true dual-SIM models. Not even sure if it will fit. There's nothing inside that would allow it just with a SIM tray. Neither the hardware nor software will work with dual-SIM.