For the Samsung NX300 I have 3 bags, each with their own purpose. I specifically buy my bags from the Photo & Film Expo that runs once a year in South Africa as the show specials are pretty good and the variety of bags is so massive you will definitely find something.
So, here are the 3 bags I have especially for my Samsung NX300 as well as links to where you can buy them.
Size is part of what the Samsung NX300 is about and you obviously want to keep the carrying size to a minimum. My first, and smallest option, is the Lowepro ILC Classic 50.
The Lowepro ILC Classic 50 holds the Samsung NX300 with one pancake lens, the flash attached with compartments for memory cards and one spare battery. I prefer to use the little pocket, marked for memory cards, for the extra battery instead and the front pocket for the memory cards. You are restricted here because a bigger lens may not fit (the 18-55mm won't) but if you want a small camera bag, even one that can fit in a woman's bag, then this is a good start. It comes with a shoulder strap.
Where to buy?
The second option, the Vanguard BIIN 10, provides a little extra room for a larger lens and gives you the option to carry the extra lens like the 18-55mm but then space becomes a little tight. The bag can easily carry two pancake lenses, one attached to the camera and one loose. It's only slightly larger width wise than the Lowepro ILC Classic 50 but still remains nice and compact.
In the pic above, the camera holds the Samsung NX300 with 30mm lens attached and next to it the 18-55mm (in its own bag). When carrying a lens the size of 18-55mm, the flash needs to be removed and stored in the top compartment with the extra battery. If you are carrying another pancake lens then you will be able to leave the flash attached to the camera. You can also use the top compartment for memory cards or use the front pocket of the bag. Comes with a shoulder strap.
Where to buy?
Loot Vanguard BIIN 10 - there is a link to the Loot website on the right side of my blog where you can sign up for free.
The next bag in my list is probably the biggest you will go unless you intend taking more than 2-3 lenses with you, then I would suggest a sling bag. The Lowepro Adventura 160 is shoulder bag that I also use for my Canon 650D.
It's spacious but not bulky and can easily hold the Samsung NX300 with 30mm and flash attached, the 18-55mm lens, an extra pancake or normal sized lens, memory cards and extra battery. The front compartment holds the memory cards and extra battery. You could probably add a lens pen and cleaning cloth as well. This is the more versatile of all three bags but definitely bigger. Comes with shoulder strap.
Where to buy?
Kalahari.com doesn't have the 160 in stock but does sell the 170, which is only a few cm bigger.
If you really want to go wild you can look at the Lowepro Slingshot 102 AW. I have its sister the 202 AW. The thing about a slingshot bag is that when you need to carry a bit more than a shoulder bag can handle, you want the ability to access your camera/s quickly without having to take the bag off of your back. You just sling the bag around, unzip the side pocket and grab your camera.
I trust Lowepro and have recommended the bags above because I use them myself. Other bags I would also recommend are Kata and Clik Elite. I had a wonderful Kata bag that was stolen and while Kata has fantastic bags like the Bumblebee (which I have my heart set on), they are a bit more expensive. If you can afford a Kata bag, definitely go for it.
Take note that the bags mentioned above for the Samsung NX300 don't have rain covers, the Lowepro Slingshot does and the Kata bags do (you may want to check this with the smaller camera bags).
Lowepro material is said to be water resistant and while I wouldn't test this theory out, I did get caught in an unexpected rain fall not so long ago and the Lowepro Adventura 160 kept my camera safe. Still, if your bag doesn't have a rain cover, keep a plastic bag in your camera bag (even if you do have a rain cover).
Vanguard also says their material is weather proof and provides a 2-year warranty.
There isn't a hard and fast rule of what camera bag is best. Just know what you want to use the bag for, check how well it's padded and secure and what it can carry. One camera bag is never enough as each one has its own place in different situations. Just start off with one that suits your needs and go from there.