Obtaining a SIM for Sierra Wireless Modem?

In an earlier posting to this forum I posed a question about data plans for Sierra Wireless GX450 modems.  I own one.

Some very knowledgeable folks responded that all of AT&T's current plans for these devices can be found here (and only here):

https://buyasession.att.com/sbd/Common/ShopRatePlans.action. 

That page directs me to this webpage: 

https://buyasession.att.com/sbd/CustNew/ShowProductInfo.action 

which asks me to enter my Sierra Wireless modem's IMEI and ICCID as a first step.

The problem is that a new modem has no ICCID number until you obtain a SIM card.  However, getting that SIM card is the goal of this process.   Naturally, the order entry process comes to a screeching halt until I enter my ICCID.

Anyone have any suggestions on where I can obtain a SIM card and data plan?