Hey danymcc,
the connector most modems do have is called CRC-9. I guess you'll find some antennas on ebay using that connector. I personally own the E122 GSM/WCDMA modem from Huawei which has this connector and you can find tons of antennas for it. But I personally would recommend you getting a CRC-9 to SMA pigtail and an SMA whatever-band-you-need antenna since you would have a solid connector on your pelican case and not just some antenna sticking out of a hole.
Hope this helps :)