Pitcher Pump Seal Kit and Washer
You can get the seal kit (PP75) by phoning Machine Mart’s Parts Dept. on 08714101270 (it’s not available online) and asking for item number ZHEBPP7501 If you only need the washer, look on eBay for: 3 1/4″ Leather syringe pump cup washer.
The Barrel Makers is another useful company, offering a range of cast-iron pumps and spare parts.
https://www.thebarrelmakers.co.uk/department/cast_iron_pumps/