If it were me: I'd remove one of my existing plugs and take it to a John Deere dealer and see if it fits one of their models listed in the description above. There do seem to be a BUNCH of different hydraulic coupler sizes out there.
Ford 3910, John Deere 420C, Kubota G32XKS, IH 2606, Bad Boy Maverick-60"
I just bought a bunch of caps and plugs for $1.99 each from Agristore. I added them to an order for other hydraulic items. If you know what size couplers you have, order by size.