In the US, we have a generic called mercaptopurine. Perhaps you could order some from here? I order from Canada a lot, never 6mp but I order Asacol generic (we don't have a generic in the US). Mercaptopurine is not too expesive but it's not cheap and you wouldn't be able to use insurance I doubt. Hopefully you're able to get some before the 25th. I have taken a few weeks off and haven't had any problems with a flare up.
There was a discussion about this a few weeks ago on the UC board:
https://www.healingwell.com/community/default.aspx?f=38&m=2192416