Portable repeaters for first responders?

Yes they can, here you go ! ! !

I am looking for the opposition of a jammer. Something that could boost a signal so that officers could use the device within an area that they normally wouldn’t receive a radio signal.

Are you looking for a non-powered repeater? Or do they have a power source? A basement is only a issue for signals if there is a lot of metal ( wires or siding ) the signal could be normally attenuated in that case. Mobile phone companies sell signal boosters but some use an internet connection.

Would something like this work for you? Incident Portable VHF/UHF Repeater by Tactical Electronics.

Here’s another example by RCA: Compact Rapid Deployment Portable Repeater