I also have a flaccid key unit where the spring and locking mechanism are not working.
It appears as though the top (key section) of the fob is a sealed unit. There is a seam where two halves of the top section join but it appears to be fused together. I could not see the spring or locking mechanism. I assume you can get a blank key recut for a lot cheaper than replacing both the key section + electronics of the bottom fob section, but I haven't checked with my dealer.