I haven't heard back from Navman here, but in case you're interested & want to follow up with NavMan OEM in the US, heres the
address for their US operation:

Navman OEM
27121 Towne Centre Drive
Suite 220
Foothill Ranch
CA 92610-2807
U.S.A
T: +1 949 461 7150
F: +1 949 461 7860

The part you want is the:

"AA004000" (OEM) "Stealth" antenna


See what you come up with price and delivery wise.
I will try to get a price here, then see which one is the cheapest.