Probably Genuine Audi is the way to go? They're probably breaking because they're cheapy aftermarket springs, they're pretty **** things.
They do a couple heavy duty options, will need careful scouring of the parts listing to find them though.
The Audi Spring codes are thus:
1BA Standard
1BB Heavy duty (20mm higher)
1BC Special purpose
1BD S4
1BE Sport
1BP Heavy duty (same height as 1BA but with limited bump)
1BT Heavy duty (7mm higher)
But theres also a weight category that goes along with it, if you get me that category from the service sticker, i'll look up a part number for you. The weight category should be something starting 0Y, like 0YA or 0YC etc.