If you're looking for a low cost/high quality serial GPS for use with the 91AD I have really enjoyed my GlobalSat BR-355 (google it) which includes the excellent SiRF Star III chipset and WAAS augmentation for better location fixes. It can be had for under $50 and works well. It does not contain a battery but I built a simple AA battery pack for it which has quite a good runtime.
-Ben Franske, K0BEN
