If you are speaking about engine oil filters, these are readily available from WixFilters.com and HastingsFilters.com and from NAPA which are mostly Wix as well.
If you are talking about hydro filters, I'd stick with Kubota.
As far as "cheaper", I don't think Kubota rips anyone off at $6.95 for a typical oil filter. A good filter, anywhere, will cost you $5. Not much savings, really.
Yes, sometimes the hydro and HST filters can be pricey at Kubota. But... in many cases, there simply isn't a true cross reference available.