Suggestions for RF meter for reading WiFi signals

Where can I buy a budget RF meter (1-5GHz), and which brand is good for that? I need it for verifying my experimental setup for radio tomography and require to read WiFi signals.

"Read WiFi signals" is a kind of ambiguous. Your question may refer to a simple power meter, a spectrum analyzer or a software defined radio receiver.

As for the budget, a SDR module like HackRF One is probably the cheapest solution covering a spectrum range up to 6 GHz.

You can get more suitable help if you explain in detail what you want to achieve.

https://www.aliexpress.com/item/4000...chweb201603_55

Above proposed budget RF meters will do the job. Both can transmit over a wide frequency range. A maybe important difference is that WinNWT4 have a non frequency selective power meter as detector.
If however actual need is just to transmit a WiFi-signal and be able to detect a level can any WiFi-dongle be used to receive/transmit signals as well as most WiFi-routers can report actual received signal level, RSSI, directly in its software.
Nice spectrum diagrams software to be used with any dongle and a PC or cellphone: https://www.metageek.com/products/inssider/
If the need is for something very portable, very budget and WiFi only, can two ESP8266 do the job Connect a small antenna loop to each unit and read/store RSSI.

"radio tomography" is for me something rather complex to measure if it includes measurements deep inside body tissues and then measuring of transmission loss between two antennas.
As long as possible do I prefer to do theoretical calculations, it is a lot less messy. It exist free body models as well as detailed tables such as this sait: http://niremf.ifac.cnr.it/tissprop/
I do also use these tables of tissue dielectric properties to make my own body equivalents, basically mixing water, sugar and adding small amount of salt and a tiny bit of sulfur to stop fermentation.
Transmission losses inside human body is rather high at 2.4GHz which can make measurements more complicated as error sources such as reflecting RF along outside of cable braid causes isolation problems.
These things are easier to handle at lower frequencies such as 433MHz and at these frequencies are losses lower and a lot of cheaper tools can be used to do measurements.
A such example is nanoVNA https://www.aliexpress.com/item/4000438307006.html
It can measure both reactiv and resistivity loss which in some cases are of big value, if not else, to be able to characterize antenna properties impact on measured result.

HFW35C RF Meter

HFW35C RF Meter
HFW35C + UBB2410 Omni Directional Antenna

Specialized Wi-Fi Meter for Measuring Higher Frequency Sources of Radio Frequency / Microwave Radiation (2.4 GHz – 6.0 GHz) includes Audio Signal Analysis. The UBB2410 antenna can be added for omnidirectional measurements.