The best unit to get is a bluetooth based GPS receiver. You can then place it anywhere and not have to worry about any cables going between the Axim and the receiver.
Try to get one based on the SiRF Star III chipset as that is one of the most sensitive GPS units out there.
You can also get a Compactflash based GPS receiver that fits into the CF slot on your Axim but since that is powered by the Axim's internal battery, you will have lees battery life.
You can refer this to the GPS forum on this site - lots of good knowledgable people there to help you.
http://www.mobilitysite.com/boards/gps-talk/
BTW, I have a GoPass GPT600 bluetooth unit that can be picked up for around $40US in the US. The current GoPass model is the GPT800.
Good luck