DFIS could probably help you out with the part, though it may not be cheap. I would not be too terribly concerned about being able to get it installed correctly. If you are careful and understand what you are doing, you will be able to get it done without much trouble and without having to remove the pump from the car. The biggest challenge will be getting the flange pulled off of its taper-fit onto the pump shaft, now that the screw holes are stripped. Do not use any prying or hammering methods that could shock and damage pump internals. You will have to use a 3-jaw puller from the outside and hope that it successfully pulls the flange off rather than continue to break off pieces of it... However, once you do get it off, getting a new one installed will not be a horrible process.
Remember that TDIs are timed with a computer, and as such they actually don't depend on all the mechanical pieces being in exactly the correct orientation. You will want it to be very close, because if it is far off then using the pump locking pin at timing belt changes will leave you with insufficient adjustment range in the slotted holes in the pump drive sprocket for properly setting the timing... but as long as it is within a few hairs of the original placement, it will not be any trouble. I would mark the shaft and the existing hub carefully, then find someone (a garage or well-equipped friend) who has an impact tool to spin the 22mm nut off for you so you don't have to wrestle with it. Then pull the flange off, transfer its mark to your new flange, and install as exactly as you can. Be careful to ensure that the flange does not turn in relation to the shaft as you tighten the nut onto it; you may want to lubricate the contact surfaces between the nut and where it touches the flange to help keep everything in proper orientation.
Once you've got everything tight, re-check your marks for correct alignment. As long as they are extremely close as far as your eyeball can see, there is no need to have exact alignment via dial gauge in the back of the pump... just remember to properly set pump timing with VCDS after getting it running, and as long as you have done everything correctly, you will have no trouble getting it set right and running great. All the precision in the timing of these pumps comes from the adjustment you make of the pulley's position relative to the hub AFTER the car is running, not from the position of the hub relative to the shaft.