The high C is a nasty thin little bugger and having them break is not that uncommon. However, the low G simply should not be breaking (well, I have heard of the Aquila Red low-G strings being rather fragile). So, if that low G is not an Aquila Red figure out what is causing it to break! (Then, it certainly doesn't hurt to apply the same medicine to the high-C, but even after you shouldn't be too surprised by occasional breakage of that one.)
I don't know of anybody selling high-C strings singly...though you might be able to figure out what gage you need and order single guitar strings of appropriate gage.
Single low-G strings are available, depending on brand.
John