umweltzone voucher

My wife is starting to panic. We are leaving this weekend to go touring around Northern Germany  for 8 - 10 weeks and know we have to get the unweltzone ticket.  I have read you can get them from garages whilst in Germany.  We are entering Germany around Essen heading for Bremen and Hamburg for starters.  Does anyone have any advice, information - good or bad- about doing this or can recommend an easy garage to get to along this route.

Thanks in advance for replies.   Peter