I think the Erazer x7831 is a rebadged MSI GT70. You can try searching replacement batteries for the latter on Amazon - I believe this is where MSI usually directs customers.
If you can find an OEM replacement, even if it costs a little extra, I recommend it. Some of the aftermarket batteries have been said to use lower quality electronics or cells.
You shouldn't have to worry about watt-hours, specifically. Try to match the original battery's electrical specs if you go with an aftermarket replacement (watts and amperage). Too much difference is not good.