Some sellers from China on ebay sell kits with PNA trademark.
They wrote in description:
"Accurate electrical models are needed of the opens and short circuits. A number of eBay sellers sell low-cost calibration kits, but these are effectively useless, as they don't have the mathematical models of the opens and shorts necessary to load the constants into a VNA, and we doubt they have optimal phase difference between open and short. This kit is different, in that it is designed, rather than just assembled. With this calibration kit, there is a support page with information on what coefficients to use for Agilent/HP,R/S,ADVANTEST,ANRITSU...".
Price is just 218 USD for SMA kit and looks good generally BUT I'm don't know quality and have no any information about this kits and where to get this calibration data.
I suppose that would be clever to ask him first for characterization equipment and what data they provide for each kit and look for some info if it possible.