USB bootloader for STM32F103CBT6 for ST-Link design

[S.Kamath]

We' re planning to implement ST-Link (USB to SWDIO) HW design using STM32F103CBT6

 microcontroller. This HW design is similar to any STM32 Nucleo board's ST-link HW design.  What I understood is STM32 controller will be having default UART bootloader only.

Questions:

1) Where can I get USB bootloader for STM32F103CBT6 ?

2) how to flash USB bootloader in STM32F103CBT6 ? Using UART interface or SWDIO or USB interface?