I was wondering where to download the U-Boot code for the TX2i.
So far as I know the code is the same for TX2i and TX2 and is determined by the L4T release. Any difference is probably in the configuration rather than the actual code. If you’ve flashed before you will have available “Linux_for_Tegra/source_sync.sh”. Using this you might try (assumes R32.1):
./source_sync.sh -u tegra-l4t-r32.1
(the “-u” is for U-Boot, a “-k” is for kernel)