I remember during my undergraduate years i took a course on antennas.
During
our labs . Our lab assistant used this device. A thin cylindrical tube.
Which glowed when it sensed RF radiation. Our professor used this device
to
show various polarizations of various antennas.

My question:
What is this device called. I wish to buy it from ebay or any other
source.
Any other suggestions shall be appreciated

I don't know what his device was , but a florescence light bulb (tube) will
glow around RF. If you excite it with a voltage just below the firing point
it will be very sensitive.