I don't know if I have seen any but I have seen laptops expand batteries via the docking slot before as an addition to the onboard one. Theoretically you could have one of those with a contained battery pack (as opposed to a slide in/snap in of a higher end/business laptop that typically have such docking slots).
Definitely never seen one in the flesh but I recall reading of one that would allow you to sub the DVD drive for a secondary battery as well. Most people doing anything like this today are probably after a secondary hard drive though so you will have to wade through all that.
Fortunately we seem to have left the days where said replacements would also warrant a fancy charger replacement usually on a price par with the laptop itself as well.
Equally depending upon the weight/volume you want to carry you could possibly chain a car adapter to a motorbike battery, or indeed a small UPS.
As far as like for like size replacements I guess you could get someone replace something that originally came with bottom rung 18650s with current high end 18650s. I don't commonly see it though, certainly not compared to tools.
