I know how that feels. There are far less specialists for older Audi's then you think. I've been in the same situation with my older A6 c5.
My advice would be to find the part number on your current ECU and try and find one with the identical part number. It will still need to be coded. I'd lok on eBay, including international sellers. This is why I've had to resort to, to get parts.
Alternatively speak to Audi to see the if they have any in stock.