I saw a nice selection of official BMW Motorrad bags on the UK Store, U.S. site should also have them listed. I always try to go for the official stuff first before aftermarket, kind of cool having it all matching...looking like it just rode right out of a catalog
Any idea of what size bag (in liters) you're looking for, or what you plan to carry?
Seen a 55 liter.
BMW branded bags are just that, you pay a premium to have their logo on things and it's not worth it. You can get better quality for cheaper from aftermarket companies who specialize on travel bags/carriers. sw-motech has a great selection of bags for a wide range of bikes. The g310r isn't out yet but I won't be surprised to see them come out with a few once the bike is released.
Before getting into the actual bag section, you may want to look at getting a luggage rack first. From the looks of that g310r seat, there isn't a lot of room to strap on the necessary bags for a road trip. something with a lot of fixation points.
Now that you mention it, I took a look at the luggage bars that comes with the g310r and they look weak. May need to replace the seat to a single rider seat and use the passenger area for the luggage rack, bypassing those bars.
BMW is going to come out with various luggage options for the g310r so you may find what you need from them. If you can replace the rear seat with a cowl, you can attach a luggage mounting plate in its place.