Sounds like you need a card with a higher voltage... or make sure you put in in the right way up

You are pushing it in all the way until it clicks aren't you? Also make sure it's not locked. Also try charging the GPS and turn it on while charging, this might make the internal voltage high enough for your card to work.
Try scavenging a card from a camera. Surely someone has one lying around.
Failing that, new Sandisk cards are around $15.