I believe it'll be something along the lines of a copper washer that goes between the fitting and the pump body. Might be semi-hard fibre, though.
First, though, you might try gently tightening it up and then spraying it down with brake clean to see if it's really coming from there or where the hose goes into it, or a seeping hose that's letting things drip dribble down the hose to the fitting and then drop off. Finding a replacement seal washer might take some doing, though, as that's not normally a seal that goes bad or deteriorates with age (from my own experiences). Probably a rebuilder that does power steering pumps would have or an older auto supply with lots of copper washer selections?
Just some thoughts,
NTX5467