Something like
this would be ideal, its actually a globalsat card and they are well rated. There isnt alot of difference between manufacturers when buying gps its all about the chipset, a good choice and the most widely available one is the sirf star IIlp chipset which happens to be the one used here. Ive ordered one from this guy and it arrived some time later after winging its way from honkong i didnt get lumbered with any import duty as it was labeled as a gift, it works flawlesly on my Axim x5 using tomtom navigator 2.
The mouse units tend to cause problems as they are serial and when you connect it the axim detects a serial connection and tries to activesync with it as if its a computer so youl cripple your axim temporarily if you plug it in without tomtom running. This isnt an issue with the compact flash units.
If you get the cf unit and find the tracking slow change the gps setup in tomtom to nema 9600, the gps unit is compatible with that standard too and it seems to track much faster in tomtom, hope this helps.