There are 3 different exterinal GPS units that work with apple products (that includes the ipad).
Bad-Elf WAAS Gps receiver that plugs in to the bottom of the ipad, is power by the Ipad battery.
GNS 5870 None WAAS Gps receiver that connect by blue tooth. Many pilots have this unit for their Ipad and flight software.
XGPS 150 I just heard about this one yesterday, It's WAAS, connects by blue tooth, has a 8.5 hour battery life, and comes with a couple of nice mounting options. It's have gotten some great reviews from the boaters, and pilots are review it right now. Note this unit also works with none apple products as well.
All of them can be found on amazon.com for 100.00 plus shipping. I plan on getting a XGPS 150