Jackall jacking system

A starting point might be the specialist groups for cars that were fitted with them. For instance, the Y Register of the MG Car Club sell a couple of information booklets, and sets of seals:

and I'm sure the subject has been well discussed on their site

I believe the Octagon Car Club also sell seals, and I'd guess NTG services may well do so. Peter Radcliffe of SVW Services sells seals for Jackalls fitted to pre-war MG saloons, and I'd guess could also point you to repairers.

I've just realised we went through all this last year - it would have helped if the OP had referred to that discussion, so I needn't have bothered checking that the site I recommended still sells seals.

Ho hum.