You need Eolys fluid which is also called PAT fluid when used by Citroen/Peugeot
As you probably already know it's not cheap!
The fluid should last for around 100,000 miles but it is also time limited at 5 years and any remaining fluid should be drained (and correctly disposed of as it is pretty nasty stuff) before 5 litres (do check this is the corrext amount) of the new fluid is poured in through the small filler that is behind a rubber flap beside your diesel fuel fuller cap.
The Peugeot stuff is usually cheaper than VW but 5 litres of PAT fluid is available at Amazon for ÂŁ115 .13 with free delivery today.
If you have run out I admit I'd perhaps be tempted to DIY but taking the greatest care with stout rubber gloves and face protection and perhaps decanting into a pouring jug first and using a funnel to avoid spillage...
What I don't know is whether or not anything has to be reset using VCDS or similar OBD diagnostic equipment after the refill.
As for finding a garage to do it, if you cannot find a knowledgeable VW specialist (and that means definitely NOT a VW dealer!) then consider asking a Peugeot/Citroën specialist.