The basic answer is probably no. If you buy a camera Canon may sell a battery for $50, but there is a version on Ebay for $12. That's what I think of with 'aftermarket'. But the bike industry, the ebike industry, is too small to support that. There may be some 'standard' designs that you could buy, probably from China. But anything that fits a frame or inside a frame is probably proprietary. There are two fairly standard batteries that use the mounts for a water bottle. One is cyclindrical, the other more squared off. You will see these on bikes from small companies.
The battery for my Prodeco X3 is designed to slide onto a frame, so it's what Prodeco offers. To work around it means connecting another battery, and setting it up some place else, or just attaching a pack to the existing mount. You want a second pack so it has to fit your mount and be fairly simple to connect.
This is why people may look at kits. On a kit there is a connector with the positive and negative, so you match that to the battery pack. Where do you put the battery? That's the problem. A manufacturer can make something very nice looking, hiding the battery in the frame or whatever.