You could try www.energy-solutions.co.uk or 01634- 290772. They supplied me with 2 VDO float-arm type water tank senders last year.
I did try recently for an oil gauge sender and although they said they could get one they said they don't deal with VDO so much now - but it might be worth a quick call just to see.
I purchased both water and fuel (diesel) tank senders from here and drive VDO gauges. I fitted 2 water and 3 diesel to 1 water gauge and 1 fuel gauge using change over switches.
The VDO water gauge is 4 - 20ma current loop and the stangard VDO fuel is for 30 - 250 ohn tank sender.