In respect of Tracta seals for the Ferret, these are not a common standard seal as the lip bears on a spherical surface. They were a specific design for that application. But when it comes to other seals on the Ferret, like, hub seal, challis internal seal, bevel box input, etc., these are to common Imperial sizes. So you really have to go to someone like Banisters, who maintain good stocks.
I use a good number of seals over a year and mostly for military vehicles and equipment over 60 years old, my local bearing supplier has a very good knowledge and just by quoting any visible number on the seal over the phone, failing that, measurements of shaft size, OD and width, can come up with a suitable item. On some sizes you have options of steel casing leather lip, or rubber coated casing and single lip, double lip with dirt seal, etc.
Hope this helps, Chris.
Richard