You can buy the tool from a specialist tool shop, but you might have to buy a universal set, cos you aint gonna know what size you need till you do it.
Ps make sure you use the right, good quailty oil for the box, and make sure you put the right amount, vital!
Do it right and the car will feel very smooth and the box will not be notchy. Thats if it aint good already!
What kind of mileage are you guys doing this at? Ive been thinking about it for sometime, but I'll definately get it done at the specialist, as I know how dodgy the threads can be on even the sump plug!!!
If my memory serves me right, it is a 12mm allen key. At least on the boxers. But I don't see any reason for a change to another dimension on the TS engines.
Btw. imho this a diy job worth attending to yourself. It may be fiddly, but so much more reason to be suspicious about the garage's performance in this area! If you diy, at least you know it has been done and with what oil! Depends on your worskshop facilities ofcourse, no fun doin' this job by the kerb!