I cannot speak for all laptops. My Acer 772G needed a battery replacement, I found one with NinjaBat company. They send the battery, I put it in. I do not know if your laptop battery is easily accessible and replaceable.
I would not limit yourself this way, in pretty much any system the battery is not that hard to get to even if it's enclosed. Due to trying to make systems slimmer and sleeker looking I can't think of a single system off the top of my head with an external battery.
Maybe a used system like a Lenovo 470 would be good if you really want an external battery. Just keep in mind that the older the system the harder it will be to find good batteries for it. Even if brand new a battery that sits for several years can go bad. I've had to toss out 4-5 batteries that were bad new out of the box after 3-4-5 years of sitting on a shelf.
I understand now; watched some videos of the new flat style batteries; looks like oem replacements are possible. In the case with tablets all the reviews I'd read from tablet replacement batteries (Samsung Tab A 10.1) said after 6 months or so they gave out. I have a basic understanding of lithium cells from e-bike project; still have some 30ah cells in my fridge, double zip locked with silica packs inside; be interesting one day to get them out and see how they've held up. I went back to pedaling, better exercise.