I have found that most of the cheap holders on ebay have interchangable phone holders.

I presume that this is so that the same suction or handlebar fixing clamp can be used with holders for different phone models.

I purchased a bicycle handlebar mount for my Nokia 5230 when I had it. That mount will also attach to a N900 mount I purchased seperately from ebay, and to a universal holder I also have.

This give you pointers for searching ebay.