ACECO FC60003 MK II and HACKRF with near field loop using sweep function
these do work and can find hidden transmitters
So hackRF one (eBay Chinese clone(hackRF is open source so these normally work fine ..mine is) £150 - original £300+) Near Field Antenna (£20-30) you probably have a PC that can run SDR software(mostly free) or better a version of dragonOS(free) (virtual machine, bootable USB) or a spare laptop as an OS or on a raspberry 4/400
ACECO FC60003 MK II is about £200 which is a good device easy to use but only does one thing well where as you can do many things with a hackRF but is more complex plus you need to use with a computer or a portapack etc