Unfortunately I don't have any experience with external antennas on USB modems, but I can recommend a Romanian producer and vendor of relatively cheap antennas, which is led by a German engineer:
UMTS-, WLAN- und DECT-Informationen, Tipps und Tricks sowie Web-Shop
They customize antennas for almost free, so you can chose cable quality and length and the appropriate connector type for your USB modem. I've already bought some WiFi-antennas from them and I was happy with their quality for those small prices.