G10 and micarta are similar in that they are both layered composite materials but have some distinct differences. I'm far from an expert, so I'm sure someone will come along and elaborate but the main difference is the material which is layered. G10 is layers of glass fiber soaked in resin, and then heated and pressed at extremely high pressure. Micarta on the other hand can be made from a few different materials such as canvas, denim, or even paper. I do not know the intricacies of micarta production but I assume it's a fairly limilar process (soak in some sort of binder, press until one solid piece) but far less extreme. For the purpose of his conversation I'm going to assume you're talking about g10 vs canvas micarta. The difference you'll see are that g10 is far tougher and more durable, but isn't as nice looking or feeling as canvas micarta (depending on your personal tastes I suppose). Micarta is going to feel softer and warmer than g10, but is also prone to staining (blood, oil, etc) and will develop some character over time as the oils from your hands sort of "season" the handles. G10 will not stain or wear nearly as fast, it's also stronger and more resistant to impact as well as the elements (hot, cold, chemicals) but has a bit more synthetic feel to it when compared to canvas micarta. All in all, they both are great materials for knife scales. For many people, canvas micarta may look a bit nicer, but g10 is far more practical.
As for where to get them, I can HIGHLY recommend Cuscadi out of Germany. Luke is a pleasure to do business with and the folks there put out an excellent product which can be made exactly to your liking with a nearly endless set of material and milling choices. They also have a cool material that they call Carbon Fiber Micarta which is a layered composite made of very tight weave cf and additional layers of glass fiber. It's basically a mixture of g10 and carbon fiber which is extremely durable and also looks really cool. I highly recommend you shoot Luke at Cuscadi an email and see what he can do for you.
The other popular option for griptilian scales are Kevin Wilkins Wilkins grips. They are purchased directly from Kevin Wilkins website and are usually only available in limited quantities in fewer material choices (predominantly g10). Since they are not made custom to order like Cuscadi, Wilkins just makes what he wants when he has time and sells what he has in stock. I personally don't have any first hand experience with Wilkins grips but there are many folks that swear by them.