Aftermarket is the only option if the whole decal sheet is gone.
However, here are a couple of tips to prevent this problem in the future.
I always scan the decal sheet when I start a new kit. Even if the decals are in good shape, if I destroy one by ham-handed application, I can print a new one with my printer and inkjet decal paper.
On old kits I always make a decal test before I wet any of the actual kit decals. On almost all decal sheets there are printed areas on the sheet that are actually decals. For instance, the product identification area is usually a decal, as is the trademark of the decal source. You can usually see the edge of the clear decal surrounding the printing to verify this. If one of these decals breaks up when you wet it, you know the sheet is bad and you have to save it.
There are products, called decal film, that coat the whole decal sheet and turn it into a single decal. These will save the sheet. You can apply it with a brush. The only downside is you need to cut each decal out seperately. But this is much easier than making a copy.