If your providers all use the same model, just get the same model as a spare, either new or used on eBay.
If your providers have several different laptops . . .
You might pick the one that seems to be best-liked and see if you can buy it new (or used on eBay).
Having the same model simplifies setup and troubleshooting, gives you a spare charger, and means that at least one provider will have a familiar laptop in case of failure.
The more providers you can get to use the same model, the easier things will be, but convincing them to standardize is about as hard as herding cats.
I just tallied laptops at my two worst offices.
One office has 30 laptops at two sites with 8 providers, split among 7 different models.
The other office has only 2 providers, but has 9 laptops split among 5 models. I suspect that a few are stashed in drawers and forgotten.
Both offices spend a lot of time and money trying to make their motley hardware work.
Since laptops aren't fixed in place like desktops, offices can even lose track of them. I had one office that didn't notice that two of laptops had been stolen until I asked why they had been inactive on my LogMeIn account for so long.
Before buying a spare, check if there's a forgotten laptop already floating around your office, or if one of your providers has a laptop that's perfectly good, but no longer used, or just needs a new battery.