Good for you for sticking up for yourself. They shouldn't tell you things can be done and then last minute retract their statements. That obviously puts you in a bind- either you live with your details not being perfect or you have to find a last minute alternative. That's crap.
I think cream would be available now. LC uses cream in their bouquets often. I would think Eastern Mexico would still be able to access them. After all, aren't calla lillies a main flower in Mexico.
Anyways, I wish you luck. I hope they learn their lesson and don't dick with you anymore and ensure it turns out perfect. Good luck.