I had an old ZTE 3G dongle lying around which I SIM unlocked, and then used that. It seems to work ok, I still have occasional dropout issues, but I think that's more on the USB side rather than compatibility, as when it works, it works fine...
I'm toying with the idea of getting a 4G one - and again planning on getting either ZTE or probably Huawei, as they are the more common brands that are likely to be supported.
I haven't come across any of the main vendors selling 3/4G sticks to use with their units, (I usually check the 'other items' on their ebay profile to see what accessories they offer etc) so assume that most of them are compatible.
How's the install going?
Marty