I would try Revival Animal Health. They are really easy to work with, good prices. I haven't tried exporting from them, but in the US, they are great:
If it doesn't work out, just ask me again and I will come up with some other ideas too.
They tylosin should be easy to get, but doxycycline has been having manufacture back order and I just checked with revival... they are still having trouble getting it. It should be back in a few months though.