Old Fluke handhelds used to have AC adapters available, such as the 8060 and D801. The manual warned against using anything but the Fluke adapter, which had a custom, extra long coax plug. That could be a problem today if you get an old meter - finding the approved adapter, and that will still probably not meet modern safety standards.
I assume newer safety standards and better efficiency made that go away. Most handhelds run so long anyway, although a few are power hungry. I use Eneloop rechargeables for all AA or AAA, and Energizer lithium in the old Fluke 8060A, even though I do have the Fluke AC adapter.