I've had good luck at places that sell heavy truck parts. Luberfiner is a brand name I found as a replacement for a Racor unit which might as well been sold on the moon for all the luck I had finding one locally. Keep in mind if you're looking for an hydraulic fluid filter, you want one that has as fine a micon rating as the Kubota.
Don't ask me how you determine the micron rating of the Kubota filter. Maybe a cross reference might have that info.