The real problem, is expecting the guy behind the parts counter to know anything besides where to look up the parts if given the year make and model, and sometimes, the serial number to boot! Unless you luck in on a place that still has one of their 'old-school' guys working. My nearest Massey dealer (two hours away) was able to get me the correct clutch plate for my 1958 Massey 35, with the 4 cylinder diesel. I would bet that they likely stock the CAV seals you need, as those pumps were used on a LOT of models...
You pretty much need to educate yourself to be able to recognize when you are looking at good parts or not. Hit up the yards you can find online. Pretty much a sure thing that they have a deal with either freight or Courier companies to ship stuff far cheaper than the unwashed masses get charged. I used to drive by a couple pretty large outfits, in Central Ab, and South Sask. Everything from burnt down last year model combines, to steel wheel tractors recovered from a shelter belt of boneyard...
On the upside, too, the Indians, are still making old Massey models, so you may be able to hook up with someone over that way that is willing to ship. My experiences have been a little bit mixed, ordered a bunch of parts via ebay, and they were shipped by Courier, who would not deliver to my PO Box (naturally enough) and ebay does not allow you to list a residential address... Stuff that was sent by mail, I have been consistently happy with! Seller refunded me when the stuff never showed up, but even had he not, it was, like, $35 for the gauge and sensors package...
Try Ali Express and search your make and model, to see what shakes loose. Don't dismiss ebay entirely, as searching there may get you a name and contact info that you can deal direct with...