If it is a conventional design oil seal, has it got a make and part number, failing that similarly to bearings you can order based on the measurements.Inside Diameter, outside Diameter and thickness, may be worth a try.
Maybe eBay with the measurements, I see they do have metric and imperial sizing seals.
I don't know where you are but if we needed and odd bearing or seal in the past there was a company in Exeter called BSL Ltd (Bearing Supplys Ltd) that could often help.
If you have a hydraulic repair centre (not the hose repair companies) near you, take the part with you and see if they can match it up…I’ve used them for all sorts of things from steering boxes to calipers