if you're referring to the prebuilt hex files distributed with the SDK, then you can find the S130 Dual Bank bootloader hex file in the <nRF5_SDK_v11_0_0 folder>\examples\dfu\bootloader\hex folder.
If you're referring to where the hex file is placed after you build it with Keil/GCC/IAR, then you can find the compiled hexfile in <nRF5_SDK_v11_0_0 folder>\examples\dfu\bootloader\pca10028\dual_bank_ble_s130\ <name of toolchain>\_build
if you're referring to the prebuilt hex files distributed with the SDK, then you can find the S130 Dual Bank bootloader hex file in the <nRF5_SDK_v11_0_0 folder>\examples\dfu\bootloader\hex folder.
If you're referring to where the hex file is placed after you build it with Keil/GCC/IAR, then you can find the compiled hexfile in <nRF5_SDK_v11_0_0 folder>\examples\dfu\bootloader\pca10028\dual_bank_ble_s130\ <name of toolchain>\_build
