You can rent a GPS. This is just one example of a site that rents GPS specific to your destination. I have no idea how the prices compare to other places, it's just an example to show that the rental service exists.
Obviously, you can also check with the rental agency to see what they charge for a GPS. Alway - always - ask if they can do better than the first price they give you. The worst they can say is no. You might get a better deal by asking at the location where you pick up the car. They are typically more flexible than the 800 reservation number.
When I went to pick up my car at the Las Vegas airport, I got upgraded to a full size for the price of a mid just by asking the nice lady that was walking me to my car. "Are there any free upgrades available?" "Sure, let's go pick out a full size car."
If you are willing to take a chance, you can find GPS's on eBay and Craigslist, but you may have pay something to have the European maps loaded.
Of course, there is always the "Buy and Return" method if that sort of thing doesn't bother you.