It's the same with your own backpacks you may find that a Kelty pack works for you, while a friend swears by Dana. Recently I found Wolfpacks, an Oregon company that makes intelligently designed packs, which my dogs find quite comfortable. They provide good instructions on how to fit the pack and they have an interesting website . In addition to the Mountainsmith pack, the most widely available pack is by Eagle Creek, but there are other companies around that make them. I have seen one by a company called Wenaha.
Many companies dealing in dogsledding materials also sell dogpacks.
Packs are sold with weight recommendations. My dogs, Maggie and Emma, weigh approximately the same amount, but they differ in height by several inches. If I had chosen their packs based solely on weight, I would have bought the same pack for both dogs. Although Maggie is quite strong, her low height prevents me from putting the same size pack on her that I use on Emma. Mail-order has taken over the outdoor goods market to a large degree, but I recommend a trip to the store with your dog to try a pack on before purchasing it.