@toxicmochii
Currently, the most expensive wolf breed is Bracchus, since it was originally a Kickstarter Backer exclusive bonus. The Generation 2-3 Pups are currently going for about 800 Moonstones each, and the item that can turn your First Generation Wolves into Bracchus can be traded, however, but it goes for upwards of ~4500 Moonstones.
That being said, there are plenty of ways to get Moonstones! Finding Wild Wolves with pretty colors is the one of the best ways I've found so far to earn a lot of Moonstones very quickly, and the easiest way to find Wild Wolves is generally to do the campaign far enough to unlock Explore Area 2 of any Pack's Territory, then start grinding! You'll also get the food you need to feed your wolves from exploring, so just keep an eye on your wolf's stamina bar, and be sure to feed them if they're getting low on stamina.
Keep in mind wild wolf encounters only have a 1/10 chance of actually rewarding a Wild Wolf, so I find it's best for my mental to treat the encounters as just another encounter, click the “Befriend” and then “Bark” or “Wag tail” buttons, and not hope for anything to happen, so I'm not terribly disappointed when they run away.
Another way to get rich, but a riskier one, is breeding wolves with pretty colors. You basically have to find two wolves with pretty colors that are compatible with eachother, do some market research, and then sell their pups! lw.fintastic.net is good for checking a Wolf's color range with another wolf, and it's a practically mandatory resource if you're planning on doing a breeding project.
Finally, you can also make and sell Custom apparel. This might actually be more lucrative than selling Wild Wolves, but it requires both an initial Moonstone investment and artistic skill, so I can't recommend it to everyone. If your Custom Apparel, or CA, sells well, however, you'll be rolling in moonstones in no time, though!