Well, the site I had linked to is the manufacturer's.
You should be able to google for a retailer or online shop near you.
In Europe that should definitely not be too difficult, I don't know about the US though.
In Germany, there's both
Brodit24 and
Brodit-Shop; both do ship internationally.
Prices are approximately 19 Euros for the car clip plus 25 Euros for the device mount.
They even offer a mounting solution for the iPad (slightly more expensive, obviously).
About the horizontal/vertical (or portrait/landscape) issue:
The one I have allows adjustment in any direction, so it's not a problem at all to put it in landscape mode.
You can adjust it so that it fits tightly enough in any case, even though they won't mention or recommend that. No worries.
However, I got accustomed to leaving my iPhone in portrait mode, even when using navigation software.
Just because an original Tomtom device displays the map in landscape mode doesn't necessarily mean that that's actually the best way to look at a map while driving...