I would recommend staying away from any amplified/powered "booster" type of device. If it isn't authorized (which only a few Andrew brand types are) and it causes interference issues with the cell site or to other customers, you have a heap of trouble on your hands.

You can get an external antenna and adapter for your USB modem. The Verizon SKU for the antenna adapter should be PCCAB-727 or you may be able to find it on the verizonwireless.com website (in accessories) and Wilson Electronics (noted below) also sells these.

As for the antenna, this may depend on how much space you have for it. A simple magnet-mount can sit on top of a metal (steel) plate/shelf/cabinet/etc. It needs to be attached to something metal as that provides a ground plane for the antenna; without a ground plane it won't work correctly. If you have the space, a yagi antenna is far more powerful, quite a bit larger, and more expensive. Remember the old TV antennas that had to be rotated for different channels? Same concept. However, a yagi that is correctly aimed can offer two to four times the "gain" of an omnidirectional antenna (the kind built into a handset or a magnet mount).

I personally use a 13db gain yagi from Wilson Electronics, I have had good results with it on a USB720 modem. http://www.wilsonelectronics.com////ViewProductB.php?ID=13

The adapter should be around $10 and the yagi around $70 to $80 (eBay, google, shop around). You'll also need a cable to connect the two which should be available wherever you would buy the yagi. The antenna has an N connector, the adapter is will be TNC, FME or Mini-UME. This can get hairy since there are no standard connectors. If you buy the antenna, cable and adapter in one place, they should be able to help you connect it together.