At the time Im reading this the OP has 6 posts so Im assuming he doesnt know he doesnt need an external GPS receiver. This also means he has a lot of reading and re-reading to do to understand what he was reading the first time.
So start reading
here and read it over and over until you understand it.
If you still want a external GPS receiever and youre looking on ebay and they are too much, then you dont need one. Cheapest I remember using was a Holux 240