OK, for the Ku Band 12 to 16GHz, I can definitely recommend the octagonal horn patented in 1978 by UK ERA, Dr.Alan Rudge. The details are in the UK Patent (now expired) 26226/78, and an article in the UK IEE (now IET) conference publication 169.
I have dimensions for 13GHz, and I made a model for a test range (for study purposes only of course) and it worked very well. Dr Rudge claimed to achieve -50dB cross-polarisation over part of the band.
The key to obtaining low CP levels is to ensure (somehow) that the E and H plane radiation patterns are as nearly identical as possible. This is possible to achieve with a horn because the currents are constrained by the walls.
At 1.8-2GHz it might be possible to construct a similar horn scaled in frequency, but the dimensions would be large. It is rare to find a design, for example a patch antenna, that has a CP level any better than about -15dB relative to the co-polar beam on boresight. It might be better to contact a company like www.tdkrfsolutions.com
Unfortunately their data sheets on the web do not quote the CP performance, usually a warning sign they are not good. If something is good, marketeers are never slow to blow the trumpet.